Jack Leach is to fly home from England's tour of South Africa after failing to recover from illness. Leach, who arrived in the country as England's first-choice spinner, has not been able to bowl a competitive delivery on the tour after being taken ill ahead of the first warm-up match in Benoni in mid-December.

Leach was also taken ill in New Zealand in November. Originally described as a bout of gastroenteritis, it now transpires the episode deteriorated to the extent that he was suffering from sepsis. He was hospitalised for several days in Hamilton and has not bowled a ball in anger since the Mount Maunganui Test, which ended on November 25. He will leave South Africa on Thursday.

Leach's situation is complicated by his long history of Crohn's disease, which requires him to take immune suppressant medication. The England management are keen not to push him too hard in his recovery phase and risk any long-term health implications. While Leach currently appears healthy, and has been able to play a full-part in the last couple of training sessions, his fitness levels have not recovered to the extent where he could be considered for Test selection.

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The England management remain hopeful that Leach will be available for England's Test tour of Sri Lanka - the first Test starts in Galle on March 19, the first tour game on March 7 - but he must be considered a doubt at this stage.

"It has been an unfortunate time for Jack with illness and since the Test series in New Zealand six weeks ago, he hasn't been able to get 100% fit," England head coach Chris Silverwood said. "This has hampered his preparation in South Africa and despite his best endeavours he is not in a position to make himself available for selection for the final two Test matches.

"He is a great lad to have around the squad and his infectious personality and popularity will be missed. However, his focus has to be getting himself better and receiving the optimum levels of recovery and treatment, which is best served back in England without any distractions.

"I have no doubt that he will return to full fitness in the medium-term and hopefully he will recover in time for our tour of Sri Lanka in March."

Leach is the third member of the tour party to return home. Rory Burns left as the second Test started after sustaining ankle ligament damage, before James Anderson suffered a fractured rib in the Cape Town Test. At least 11 members of the playing squad - and around half-a-dozen of the non-playing tour party - have also suffered from a sickness bug that decimated the team during the first Test and resulted in Ben Stokes referring to this, only partly in jest, as "the cursed tour".

England are unlikely to call for further back-up for Leach. They still have two specialist spinners in the squad - Dom Bess and Matt Parkinson - and there is a possibility neither will play in the final Test in Johannesburg. Bess, Leach's deputy at Somerset, performed a good holding role in Cape Town.

Looking ahead to Sri Lanka, though, this episode may further incentivise the England management to repair the relationship with Moeen Ali. Moeen, who is currently taking an extended break from Test cricket, has previously intimated that he was minded to "see out" his PSL contract, which would exclude him from the Sri Lanka tour, but there is little doubt the England management will try to coax him back to the Test team.

He is understood to maintain a good relationship with both Silverwood and England captain Joe Root, and has been invited to take part in some mentoring an England Lions training camp at Loughborough this week.

Moeen and Leach were the equal highest wicket-takers when England won in Sri Lanka little more than a year ago but, at this stage, there seems a possibility that neither they nor Adil Rashid, who was the third member of the spin attack, will return.