Prince Harry's shock revelation that he killed 25 Taliban fighters in Afghanistan should increase security concerns ahead of his trip to Nigeria, a former head of royal security has warned. The Duke of Sussex and his wife, Meghan Markle, will visit Nigeria in May for talks on the Invictus Games.

Harry is expected to make the trip after a service at St Paul’s Cathedral to mark the 10th anniversary of the games. Nigeria was among the countries which participated in last year's event, while Meghan Markle has recently claimed that tests revealed she is of 43 percent Nigerian heritage.

Dai Davies, a former Head of Royal Protection, suggested Harry's claim to have killed 25 in Afghanistan raises alarm bells from a security perspective.

He told Express.co.uk: "Just over half of [Nigeria's] population are Muslim. That gives me concern because of what Harry said in his book."

Harry served in Afghanistan as an Apache helicopter gunner in 2012-13 and was deployed on active duty in Helmand province in 2007-08.

The Duke of Sussex writes in his memoir, Spare, that during his two tours with the British Army he found himself "plunged in the heat and confusion of combat" and didn't think of those 25 as people. He goes on to say: "They were chess pieces removed from the board".

Harry's claim sparked criticism from military figures, royal commentators and veterans' relatives. Former UK Defence Secretary Ben Wallace told LBC's Nick Ferrari: "I frankly think boasting about tallies... distorts the fact the army is a team game. It's a team enterprise. It's not about who can shoot the most."

Mr Davies told Express.co.uk some parts of Nigeria are extremely dangerous and pointed to Global Peace Index's measure of safety, security, conflict and militarisation around the world. It places Nigeria 144th out of 163 countries - meaning only 19 countries in the world are more dangerous.

Britain's Foreign, Commonwealth and Development Office (FCDO) advises against all travel to several parts of Nigeria. The African country's military has been fighting a deadly war against Islamic extremists in the country’s northeast since 2009.

A former Divisional Commander at the Metropolitan Police, Mr Davies said: "Threats will come at [Harry], particulary because of the controversy over killing Muslim brothers."

But risks entering the Sussexes' security team's calculations should also include kidnapping, terrorism, a lone attacker and concerns about disease, according to Mr Davies.

Mr Davies said: "I would have grave concerns unless it was properly prepared with all contingency plans in place... I would look at the circumstances of this visit and ensure it's as safe as possible."

He added: "I would want to know the rationale for this visit to what is a very dangerous place... I'm all for supporting servicemen. The Invictus Games is fantastic and I would give [Prince Harry] credit for that.

"But I would want to know why he wants to go to Nigeria. It's a brave man who can say it's a safe place."