A South African mercenary who helped the Nigerian army recapture huge amounts of territory from Boko Haram insurgents has accused the country’s government of squandering the gains it made with his help.

Colonel Eeben Barlow, a veteran commander in the apartheid-era South African Defence Forces, led a team of mercenaries who secretly trained up an elite Nigerian strike force back in 2015.

Hired by then President Goodluck Jonathan in the wake of the Chibok schoolgirl abduction, the mercenaries were credited with driving Boko Haram out of most of their strongholds in north-east Nigeria.

But after just three months, their contract was cancelled by incoming President Muhammadu Buhari, a former general, who told The Telegraph at the time that the Nigerian army should be able to do the job alone.

Col Barlow has now broken his silence to accuse Mr Buhari of needlessly prolonging the war by dispensing with the mercenaries’ expertise. In a posting on Facebook, he said: "It is sad that the President preferred defeat above victory, as soldiers can only do what they are trained, equipped, and led to do.

"Do it poorly, and they die."

He added: "Many of the men we trained… have remained in contact with us, pleading for our return to Nigeria. They have also told us that they have been used to a point of exhaustion."

While Col Barlow’s comments could be seen as "sour grapes" at losing his contract, they echo wider concerns that Boko Haram is now regaining strength.

Kidnappings and killings have continued on a large scale, despite Mr Buhari’s claim last New Year to have crushed the group in their "last enclave" in Nigeria’s Sambisa Forest. An Isis-alled Boko Haram faction has also mounted 17 attacks on army bases this year, with up to 100 soldiers reported in one assault in the remote desert town of Metele on November 18.

Col Barlow added: "Raids on villages and the slaughter and kidnapping of the innocent and defenceless has continued—and in some instances intensified—under the reign of the President Buhari’s government."

He said that back in 2015, his firm had warned the Nigerian government that unless the strike force was allowed to decisively "annihilate" Boko Haram, the group would spread  into neighbouring Chad, Niger, and Cameroon, where it has since become well-established.

"These intelligence warnings were all rejected in favour of a false belief. Neighbouring countries were advised not to listen to us as it was claimed to be a cheap attempt to ‘get a contract’," he added.

Col Barlow was a founder of Executive Outcomes, a private military company made up of many ex-members of South Africa’s security forces. One of the first modern "private armies", in 1995 it successfully helped the government of Sierra Leone defend itself against the rebels of the Revolutionary United Front, notorious for chopping off the arms of their enemies.

His new company, known as Specialized Tasks, Training, Equipment and Protection, is thought to have sent around 100 men to Nigeria.