Britain is expected to respond soon to Moscow's refusal to hand over the ex-KGB agent accused of murdering Alexander Litvinenko.
Foreign Secretary David Miliband said the Government had been "seriously" considering all of its options after Russia rejected British demands for the extradition of Andrei Lugovoy.
There was speculation that a statement to Parliament, due this week, could come on Monday afternoon.
Mr Litvinenko, a former Russian security agent who fled to Britain, died in a London hospital in November from a fatal dose of the extremely rare radioactive isotope polonium 210.
He was a fierce critic of the Russian regime and had close links with a network of expatriate dissidents, both in the UK and New York.
The Crown Prosecution Service (CPS) announced in May that it would seek the extradition of Lugovoy to face trial for Mr Litvinenko's murder.
Downing Street last week described Moscow's refusal to co-operate as "extremely disappointing".
Mr Miliband on Sunday kept open the option of expelling Russian diplomats as he insisted that Britain would defend the "integrity and independence" of its judicial processes.
"A very serious crime was committed in the streets of London," he said. "We have a judicial process that must be seen through and I don't want to say any more at the moment other than we are considering seriously all of our options.
"We've got a judicial process, with integrity and independence and we will defend that."
