Sir Keir Starmer has led tributes to a member of the British armed forces who has been killed in Ukraine.

The prime minister expressed his “deepest sympathy and condolences” for the services member, who died as he watched the testing of a new defensive capability, according to the Ministry of Defence.

The ministry said: “It is with deep regret that we must announce that a member of the UK armed forces died in Ukraine this morning, Tuesday December 9.

“He was injured in a tragic accident while observing Ukrainian forces test a new defensive capability away from the front lines.

“The family has been notified and our thoughts are with them at this sad and difficult time.”

The UK has previously acknowledged that a “small number” of military personnel are in Ukraine, mainly providing security for the British diplomatic presence and supporting the Ukrainian armed forces. The government has never said how many that is.

Sir Keir wrote on social media: “My deepest sympathy and condolences to the family of the member of our armed forces who sadly lost their life today.

“Their service and sacrifice will never be forgotten.”

Defence secretary John Healey said in a social media post: “Devastated by the death of a UK service person in Ukraine.

“My thoughts are with their family, friends and colleagues as they grieve for a loved one. Our hearts go out to them.”

Shadow defence secretary James Cartlidge said: “I am deeply saddened to hear the tragic news that a member of the UK armed forces died this morning in Ukraine.

“My sincere condolences to his family and friends at this difficult time.”

The British armed forces have a five-week intensive programme in the UK that trains Ukrainian troops to become “battle-ready”.

The death is not being put down to hostile fire, the Press Association understands.