Britain has held back from an immediate escalation of the round of tit-for-tat reprisals with Russia in the wake of the Salisbury poison attack.
But Prime Minister Theresa May told a meeting of the National Security Council (NSC) that further measures were under active consideration and she was ready to deploy them “at any time”.
Strict checks by border officials on private flights have been introduced as the UK increases efforts to monitor and track those entering the country.
There had been speculation the NSC might agree new retaliatory measures after Moscow’s response to the UK’s first round of sanctions went further than Mrs May’s expulsion of 23 suspected spies from the Russian embassy.
As well as throwing out 23 UK diplomats, Moscow ordered the British Council to cease activities in Russia and withdrew permission for the reopening of the St Petersburg consulate.
The Russian “undeclared intelligence officers” left the country’s consulate today, six days after Mrs May gave them a week to pack their bags and leave the UK.
A procession of vehicles took a number of individuals away from the gated Kensington Palace Gardens complex close to the Russian Embassy in west London.
Children, suitcases, bags and pet baskets were loaded into the three cars, five people carriers and three small-sized coaches, which left the west London complex shortly after 10am.
The NSC meeting heard action had been taken over the past week to tighten checks on private flights coming into the UK and to prepare legislation, which will allow the authorities to target the assets of foreign nationals linked to human rights abuses.
“The Prime Minister reiterated that we will freeze Russian state assets wherever we have the evidence that they may be used to threaten the life of property of UK nationals or residents,” said the PM’s spokesman.
“And led by the National Crime Agency, we will continue to bring all the capabilities of UK law enforcement to bear against serious criminals and corrupt elites.
“The Prime Minister said that there are other measures the Government and security officials are actively considering and stand ready to deploy at any time.”
The spokesman declined to comment on the decision of the president of the European Commission, Jean-Claude Juncker, to write to Mr Putin to congratulate him on his re-election as president.
Without mentioning the 4 March nerve agent attack that has left ex-spy Sergei Skripal and his daughter Yulia in a critical condition in hospital and injured a policeman who came to their aid, Mr Juncker called in his letter for “positive relations” between the EU and Russia.
Meanwhile, Labour leader Jeremy Corbyn has said he would still do business with Mr Putin despite “all fingers” pointing towards Russia being responsible for the Salisbury spy incident.
He told Radio 4’s World At One: “All fingers point towards Russia’s involvement in this, and obviously the manufacture of the material was undertaken by the Russian state originally.
“What I’m saying is the weapons were made from Russia, clearly.
“I think Russia has to be held responsible for it but there has to be an absolutely definitive answer to the question where did the nerve agent come from? I asked the Russians be given a sample so that they can say categorically one way or the other.”
Mr Skripal, a former double agent, and his daughter are still fighting for their lives after being exposed to Novichok in the Wiltshire city.
Mr Corbyn maintained there had to be a relationship with Russia and said he would still “do business” with president Putin if Labour came into power.
“Would I do business with Putin? Sure. And I’d challenge him on human rights in Russia, challenge him on these issues and challenge him on that whole basis of that relationship,” he told the BBC.
Foreign Secretary Boris Johnson accused Russia of trying to conceal “the needle of truth in a haystack of lies” over the case - after Mr Putin dismissed the idea of Russian responsibility as “nonsense”.
Writing in the Daily Telegraph, the Foreign Secretary said the use of a nerve agent against the Skripals was “very deliberate”.
“As Ken Clarke pointed out in Parliament last week, the obvious Russian-ness of the weapon was designed to send a signal to anyone pondering dissent amid the intensifying repression of Mr Putin’s Russia,” he wrote.
“The message is clear: we will hunt you down, we will find you and we will kill you – and though we will scornfully deny our guilt, the world will know that Russia did it.”