The blue riband competition of the European rugby union club calendar, which now also includes the cream of South African sides, kicks off this weekend with the keenly anticipated first series of pool matches in the Investec Champions Cup.

This season sees a change to the format of recent years with the competing teams split into four pools of six and each playing two games at home and two away against four different clubs. There will be no fixtures between sides from the same league.

At the conclusion of the pool stage, the four highest-ranked clubs from each pool will qualify for the round of 16 and the clubs ranked fifth in each of the pools will qualify for the knockout stage of the Challenge Cup.

BBC Sport assess the prospects for each of the four Irish provinces in the forthcoming campaign.

Leinster

Leinster's impeccable pedigree as one of the powerhouses of European rugby and a squad packed with Ireland internationals makes them one of the perennial favourites to lift the silverware, even if they are without recently retired out-half Johnny Sexton.

Their consistently high performances have regularly propelled them to the latter stages of the competition in which they last tasted victory in 2018.

The four-time winners missed out on a top seeding this time round as a result of their 2022-23 United Rugby Championship semi-final exit to Munster and an agonising Champions Cup final defeat by La Rochelle for the second year in a row.

Leo Cullen's charges have been handed a tough draw in Pool 4 against Ronan O'Gara's double title winners, English heavyweights Leicester Tigers and Sale Sharks, plus Stade Francais. The Stormers, based in Cape Town, complete the group.

Leinster have been gradually integrating their international contingent back into their matchday squads after the World Cup, giving them their customary enviable strength in depth.

The recent arrival of South Africa's World Cup winning-coach Jacques Nienaber as a senior member of the coaching team offers a further significant boost to a team who will seek to improve on their record of being runners-up in three of the last five seasons.

Since losing to Glasgow in their URC opener, they have seen off Munster, Scarlets, Edinburgh and the Sharks at home and the Dragons and Connacht away to take their familiar place at the top of the table.

Fly-half Ross Byrne is an injury worry ahead of their trip to France to face old foes La Rochelle in their opening game, after missing their last-gasp weekend victory over Connacht with an arm injury.

Stade Francais and La Rochelle sit fifth and ninth respectively in the French Top 14 standings, with Sale top of the English Premiership table despite losing heavily to Harlequins on Friday. Mid-table Leicester Tigers will be encouraged by an emphatic win over Newcastle on Sunday.

Leinster's fixtures: 10 December: La Rochelle (away), 16 December: Sale Sharks (home), 13 January: Stade Francais (home), 20 January: Leicester Tigers (away).

Munster

Munster are regarded as one of the 'big beasts' of European competition and will be fancied to progress far, especially having shown their ability to perform under pressure and rise to the big occasion in winning the URC title last season.

Winners of Europe's premier club competition in 2006 and 2008, they were eliminated by the Sharks at the last-16 stage last term, having reached the quarter-finals the previous season.

An impressive record also includes three successive semi-final appearances between 2017 and 2019 and their domestic success last year saw them claim a berth as top seeds for this year's tournament.

This campaign's Pool 3 opponents include 2020 champions Exeter, who they beat at the last 16 stage in 2022, and for the second year in succession at the pool stage, Northampton Saints.

Graham Rowntree's men will also face three-time champions Toulon and first-time opponents Bayonne. In-form Glasgow Warriors are the sixth team in the group.

The Irish side have been boosted by the news Springbok second row Jean Kleyn has signed a new two-year contract, although fellow South Africa World Cup winner RG Snyman, departing at the end of the campaign, is likely to be ruled out until March with a chest and shoulder injury.

Joey Carbery is trying to overcome a wrist problem, while Peter O'Mahony (shoulder) and Jack O'Donoghue (ankle) are set to be unavailable for the opening encounter against Bayonne, but could be fit in time to face Exeter the following week.

Munster's squad also includes Irish internationals of the quality of Jack Crowley, Conor Murray, Tadhg Beirne and Dave Kilcoyne.

They currently sit fourth in the URC standings after home wins over the Stormers, Dragons, Sharks and Glasgow, accompanied by defeats on the road by Irish interprovincial rivals Leinster and Ulster and a drawn game at Benetton.

The form book suggests a tough trip to Toulon in January given the French outfit presently lie second in the Top14, with first-round opponents Bayonne eight places worse off.

Another formidable away day at Exeter awaits, while Northampton will kick off their bid for continental success with confidence high following their 18-12 triumph over Saracens at the weekend.

Munster's fixtures: 9 December: Bayonne (home), 17 December: Exeter (away), 13 January: Toulon (away), 20 January: Northampton (home).

Ulster

Ulster have been handed a tough draw as they must do battle in this season's tournament with French Top 14 champions Toulouse, Johann van Graan's Bath, Harlequins and Racing 92, with Cardiff completing Pool 2.

The 1999 champions have so far failed to replicate that historic achievement of almost a quarter of a century ago, going closest in 2012 when they lost to Leinster in the decider.

Leinster also accounted for Dan McFarland's team in the last 16 last season, the same stage in which Ulster departed in 2021 when they lost on aggregate by a mere point to eventual champions Toulouse after a two-legged tie.

The 2019 and 2020 quarter-finalists suffered disappointment on the domestic front in May when they went out to Connacht in the URC quarter-finals despite enjoying what is normally a crucial home advantage.

The northern province shook off that sense of having underperformed by making a promising start to the URC season, defeating the Lions, Munster, Bulls and Zebre, but have now lost three of their past five games, suffering reverses at Connacht and Glasgow, and most recently against Edinburgh in Belfast.

Doubts surround the fitness of seasoned Irish internationals Iain Henderson and Rob Herring for the testing opener against Bath at the Rec, while in-form flanker Dave McCann is ruled out for the medium term.

The arrival of double South African World Cup winning prop Steven Kitshoff represents a major fillip.

Wings Jacob Stockdale and Rob Baloucoune, Ireland centre Stuart McCloskey and forwards Tom O'Toole and Nick Timoney are among the most notable names in the squad.

Adopting a new brand of attacking rugby this year, Ulster will look for improvement this time out, having managed only one win from their four pool outings in 2022-23.

Bath's win over Exeter on Saturday moved them second in the Premiership, level on points with pacesetters Sale, while third-placed Harlequins will be buoyed by their demolition of the Sharks.

Racing sit atop the Top 14 despite defeat by Clermont on Saturday, with five-time Champions Cup winners Toulouse currently six points in arrears in sixth.

Ulster's fixtures: 9 December: Bath (away), 16 December: Racing 92 (home), 13 January: Toulouse (home), 20 January: Harlequins (away)

Connacht

Connacht shed any lingering notion of being the poor relation of the Irish provinces with their semi-final appearance in last season's URC and a positive start to the new campaign which saw them emerge as the early table-toppers.

Peter Wilkins' side have won four of their first seven games to sit eighth in the table, notable victories to date coming at home to Glasgow, Ospreys and Ulster and away to the Sharks. Their losses have been away to Edinburgh and the Bulls, and then agonisingly to Leinster.

The western province return to the Champions Cup for 2023-24 after one season in the Challenge Cup in which they exited to Benetton in the last 16.

Drawn in Pool 1, they will come up against English champions Saracens, Bristol Bears under their former coach Pat Lam, Bordeaux Begles and Lyon. The Bulls make up the group.

Connacht will aim to make best use of their Galway fortress, with Leinster's late narrow win at the Sportsground on Saturday ending their unbeaten run on their home patch in 2023 up to that point.

Ireland wing Mack Hansen made his first appearance of the domestic season against their interprovincial rivals at the weekend after returning from the World Cup with an ankle injury.

Midfield charger Bundee Aki's return has been on hold however as he continues his rehabilitation from a hamstring issue. His fitness will be reassessed before Saturday's visit of Bordeaux.

Saracens are fourth in the Premiership despite their five-match winning sequence coming to an end against Northampton, while Bristol warmed up by comprehensively seeing off Gloucester.

Bordeaux are mid-table in their domestic league and Lyon two off bottom spot.

Connacht's fixtures: 8 December: Bordeaux (home), 16 December: Saracens (away), 13 January: Lyon(away), 19 January: Bristol Bears (home)