LONDON- Britain revised its Russian sanctions guidance on Friday to make clear that British entities cannot help Moscow sell off its gold reserves in an attempt to evade the punitive measures designed to lock it out of international financial markets.

Britain has expressed concern that Russia could be using gold transactions to evade sanctions launched in coordination with allies after Moscow led an invasion of Ukraine.

British entities are banned from undertaking transactions with the Russian central bank, finance ministry or wealth fund under a Feb. 28 announcement.

The online government guidance on how to implement that ban was updated on Friday, with the following summary:

"Guidance updated to clarify that the prohibition on providing financial services for the purposes of foreign exchange reserve and asset management also applies to transactions involving gold."

"It is prohibited to provide financial services to carry out transactions with the Central Bank of the Russian Federation involving its gold," the guidance document said.

Prime Minister Boris Johnson said on Thursday there was evidence Russia was trying to get round the sanctions using gold, and that regulations would be tightened.

Russia could seek to monetise it gold reserves and access foreign currency by selling off bullion into the world market.

On Thursday, the U.S. Treasury Department issued guidance on its website warning that gold-related transactions involving Russia may be sanctionable by U.S. authorities.

(Reporting by William James; editing by David Milliken and Kate Holton)