Mr Medvedchuk is a close friend of Putin, who is godfather to his daughter.

He led the most prominent pro-Russian political coalition, Opposition Platform – For Life until he was arrested and charged with treason in May 2021.

The charges relate to alleged sale of military secrets to Russia and exploitation of the natural resources of occupied Crimea. He denies the charges and maintains the arrest was politically motivated.

He was under house arrest in Ukraine when Russia invaded on February 24, but escaped and disappeared soon afterwards.

He was recaptured by Ukraine’s security service on April 12, and Mr Zelensky immediately suggested exchanging him for Ukrainian prisoners.

In a video posted by the SBU at around the same time as Russia broadcast the statements from Mr Pinner and Mr Aslin, Mr Medvedchuk said: “I want to appeal to the president of the Russian federation, Vladimir Putin, and the president of Ukraine, Volodymyr Zelesnky, with a request for the Ukrainian side to exchange me for defenders of Mariupol and civilians who are there now and do not have the possibility to safely leave via a humanitarian corridor.”

It was not immediately clear whether Russia or Ukraine released the respective video appeals first.

Mr Pinner, who served in Bosnia and Northern Ireland with the British Army, and Mr Aslin, who previously fought against Islamic State with the YPG, fought with their unit in defence of Mariupol from the beginning of the war.

They were among several hundred soldiers who surrendered last week, saying they had run out of ammunition and food following more than six weeks of siege.

Russian television has falsely described them as “British mercenaries”.

Both men are enlisted regular members of the Ukrainian armed forces who joined before the current war and are entitled to treatment as prisoners of war under the Geneva convention.

‘Quick resolution’

Mr Pinner’s family said in a statement released by the Foreign, Development and Commonwealth Office that they wanted to make clear he was not a mercenary and called for a “quick resolution” so he could return to them.

They said: “Shaun was a well-respected soldier within the British Army serving in the Royal Anglian Regiment for many years. He served in many tours including Northern Ireland and with the United Nations in Bosnia.

“He progressed into the Ukrainian Marines as a proud member of his unit. At the end of 2022, his three-year contract is due to end and he was planning to enter a humanitarian role within Ukraine.

“Our family is working with the Foreign Office along with the family of Aiden Aslin to ensure their rights as prisoners of war are upheld according to the Geneva Convention.”

They added: “Shaun is a funny, much-loved, well-intentioned husband, son, father, brother and friend to many.

“Our hearts go out to all those caught up in this horrific conflict.”

The FO has been in contact with the families of both soldiers to support them but has declined to comment on the case.

A Whitehall source said the Government condemned the exploitation of prisoners of war for political purposes and called on the Kremlin to treat all POWs humanely.