Russia must not use UK citizens as “pawns in diplomatic chess” the foreign secretary has said, after the arrest of joint US-UK national Paul Whelan.
Jeremy Hunt said he was “extremely worried” about the former US Marine, who has been detained on suspicion of spying.
He added that “every support” is being given to the 48-year-old.
Mr Whelan’s family said he was attending a wedding, while Russia said he was “caught spying” in Moscow.
Mr Whelan was born in Canada to British parents and moved to the US as a child. He now lives in Michigan and is director of global security for automotive components supplier BorgWarner.
He has been visiting Russia for business and pleasure for more than a decade, his brother said.
His twin David said he had been told his brother was fine and looking healthy, after US ambassador to Russia Jon Huntsman visited him this week.
Mr Hunt said: “Individuals should not be used as pawns of diplomatic leverage. We need to see what these charges are against him and understand whether there is a case or not.
“We are giving every support we can, but we don’t agree with individuals being used in diplomatic chess games.
“Because it is desperately worrying, not just for the individual but their families, and we are all extremely worried about him and his family as we hear this news.”
He added that the UK had not yet had access to Mr Whelan and that the US was leading on this case.
BBC diplomatic correspondent James Landale said one Russian news agency suggested Mr Whelan was found in possession of a memory stick, allegedly containing the names of intelligence officials.
He also said there had been speculation Mr Whelan was detained so he could be exchanged with Maria Butina, a Russian gun rights activist who was jailed in the US last month.
Russia and America have traded spying allegations at regular intervals since the Cold War.
Mr Whelan travelled to Russia on 22 December and planned to fly home on 6 January, but was arrested in Moscow on 28 December.
Under the charge of espionage, he could face up to 20 years in jail if found guilty.
His lawyer, Vladimir Zherebenkov, said he has appealed to the court against the detention of his client and requested he is released on bail. He added that Mr Whelan remained in good spirits.
A Foreign Office spokesman said: “Our staff have requested consular access to a British man detained in Russia after receiving a request for assistance from him.”
RTE reported that Mr Whelan also holds an Irish passport and had sought Irish consular assistance.