A transatlantic probe was under way into claims that a member of an alleged Russian spy ring held a fake British passport.
Tracey Lee Ann Foley was one of ten people arrested in the US by the FBI for allegedly serving for years as secret agents of Russia's intelligence service, the SVR, with the goal of penetrating US government policymaking circles.
The alleged spy ring were involved in long term "deep cover" assignments, the US government has said.
US Department of Justice papers said Foley travelled on a "fraudulent British passport prepared for her by the SVR". She was arrested in Boston on Monday.
The Foreign Office said: "We are aware that the indictments state that one of the accused has travelled on a UK passport.
"We will be investigating this fully with the US. We are establishing the facts so it would be wrong to comment further at this stage. We remain confident that the British passport is one of the most secure documents of its kind - fully meeting rigorous international standards."
The case echoes revelations earlier this year in which forged British passports were allegedly used by Israeli agents in the killing of Hamas leader Mahmoud al-Mabhouh in Dubai.
Russia's foreign minister said Moscow was waiting for a US explanation about the arrests of the alleged Russian spies.
The 10 arrested are accused of conspiracy to act as unlawful agents of a foreign government. Eight also face a charge of conspiracy to launder money.
An 11th suspect named "Christopher R Metsos" was arrested on Tuesday in Cyprus.
