Foreign Secretary Liz Truss appeared to give Britons licence to join up when she said on February 27: "Absolutely, if that's what they wanted to do."

Prime Minister Boris Johnson's office, Defence Secretary Ben Wallace and the head of the armed forces have all disowned Truss's comment.

But Ben Grant, 30, who served for five years as a Royal Marines commando, is among seven British former soldiers who arrived in Ukraine over the weekend, The Guardian reported.

His mother, Helen Grant, is a Conservative MP and former minister who is Johnson's special envoy on girls' education.

Ben Grant, a father of three, said he had not informed his mother before going, following a stint working in Iraq as a private security contractor.

Speaking to The Guardian in Lviv railway station before boarding a train for Kyiv, he said he chose to go after viewing TV footage of a bombed house in Ukraine where a child could be heard screaming.

"In total we have another 100 people coming, so this will be really good when they all get here -- different backgrounds, some of them are very, very specialist," he said.

Grant said he was undeterred by Russia's warnings that "foreign mercenaries" taken captive by its forces in Ukraine would not be treated as prisoners of war.

"If it comes to me being able to end my life before getting captured, I probably would do that," he said. "I mentally prepared myself for that, but let's hope it doesn't come to that."

While Grant's group are ex-servicemen, The Sun newspaper reported that a 19-year-old member of the Coldstream Guards regiment was among up to four missing British soldiers believed to have gone to Ukraine.

The Coldstream Guards provide protection for Queen Elizabeth II at Windsor Castle, west of London. They are the oldest continuously serving regiment in the British Army and fought in the 1853-56 Crimean War.

- Not in it for a selfie -

In parliament on Wednesday, Wallace -- a former Scots Guard -- said any serving personnel who have gone to Ukraine "will be breaking the law and they will be prosecuted when they return for going absent without leave or deserting".

For other Britons, the defence secretary said, "We strongly discourage them from joining these forces."

"The Ukrainians are very clear: you turn up, you are in it for the whole game. You are not in it for a selfie and six weeks, you are in it for real," he said.