Britain's ambassador has issued a direct challenge to Russia over its refusal to extradite the man accused of killing Kremlin critic Alexander Litvinenko in London.
Russia has refused to turn over former KGB officer Andrei Lugovoy on the grounds that its constitution forbids extradition of Russian citizens.
But Sir Anthony Brenton said in an interview published on Monday that Russia could get around the ban if it wanted to co-operate.
"Russia's constitution, like those of other states, is clearly capable of interpretation in the light of circumstances," he said in the interview with the Interfax News Agency and Kommersant newspaper.
The ambassador noted specific sections of the constitution that were routinely violated in Russia.
Russia's prosecutor general was scheduled to address Russia's co-operation with Britain in the Litvinenko case at a news conference on Monday. The case was also expected to be discussed at a meeting of European Union foreign ministers in Brussels.
Gordon Brown has also renewed his demand for Russia to hand over the prime suspect in the Litvinenko murder - describing the situation as "intolerable".
The Prime Minister insisted Russia has a "responsibility" to extradite Lugovoy so he can stand trial for the crime in the UK.
Speaking during his first regular press conference as PM on Monday, Mr Brown said it was "very important" that the world understood the seriousness of the situation.
Lugovoy is accused of poisoning Litvinenko, a former KGB colleague, with radioactive polonium at a London hotel on November 1 last year. He has denied any involvement.
