Theresa May has backed away from new reprisals against Moscow following this month's nerve agent attack in Salisbury, focusing instead on targeting Vladimir Putin's associates in Britain, including new security checks on private flights.

Mrs May decided not to escalate the tit-for-tat diplomatic battle with Moscow that saw 23 Russian officials – described by Downing Street as "undeclared intelligence officers" – expelled from Britain on Tuesday with their families, The Financial Times said.

Instead Mrs May decided to adopt a longer-term approach, toughening Britain's sanctions on associates of the Putin regime in London and working with western allies to put pressure on Moscow. "Other measures are being actively considered and we stand ready to deploy them at any time," said a spokesperson for Mrs May.

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Mrs May's hopes of galvanising western support against the Kremlin suffered a setback when President Donald Trump telephoned Mr Putin to congratulate him on his re-election as president and to discuss a possible meeting. Mr Trump did not mention the Salisbury attack, the White House said.

Mrs May is hoping for a show of European unity at a Brussels summit on Thursday, although Britain is not pushing for new sanctions to be adopted by a bloc that is divided on how to deal with Moscow.

In a sign of those tensions, Jean-Claude Juncker, European Commission president, irritated Downing Street by sending Mr Putin a letter of congratulation on his poll victory.

Mr Juncker offered the Russian president "every success in carrying out your high responsibilities", saying that he would be a "partner" with Moscow in maintaining "the security of our continent".

The bloc's foreign ministers stopped short on Monday of fully endorsing Britain's claims about Moscow's culpability. An EU diplomat said: "Until now nobody is talking about additional sanctions."

Downing Street said Britain was already taking tough action against Russia including "dismantling its espionage network" and tightening sanctions on the assets held in Britain by allies of Putin accused of corruption or human rights abuses.

Security checks on individuals and their possessions would be stepped up at the airports used by Russian oligarchs, a move intended to disrupt the flow of illicit cash and suspicious materials into Britain.