THERESA May has held back from escalating the tit-for-tat row with Russia over the Salisbury chemical attack as the 23 "undeclared intelligence officers" headed back to Moscow.

The Prime Minister told a meeting of the National Security Council[NSC] further measures against the Kremlin were under active consideration and that she was ready to deploy them "at any time" but declined to instigate further retaliatory measures.

Earlier, a procession of vehicles took a number of individuals away from the gated Kensington Palace Gardens complex close to the Russian Embassy. Children, suitcases, bags and pet baskets were loaded into three cars, five people carriers and three small-sized coaches, which left the west London complex shortly after 10am.

READ MORE: Crisis looms as Holyrood set to pass its own alternative Brexit Bill

The NSC meeting heard that action had been taken over the past week to tighten checks on private flights coming into the UK and to prepare anti-money laundering legislation which would allow the authorities to target the assets of foreign nationals linked to crime and 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," explained Mrs May’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 Jean-Claude Juncker, President of the European Commission, to write to Vladimir Putin to congratulate him on his re-election as President.

Without mentioning the March 4 nerve agent attack which has left the former spy Sergei Skripal, 66, and his daughter Yulia, 33, 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.

He told Mr Putin: “Our objective should be to re-establish a co-operative pan-European security order. I hope that you will use your fourth term in office to pursue this goal. I will always be a partner in this endeavour. I wish you every success in carrying out your high responsibilities.”

Ashley Fox, leader of Conservative MEPs, branded Mr Juncker’s letter a disgrace.

“To congratulate Vladimir Putin on his election victory without referring to the clear ballot-rigging that took place is bad enough but his failure to mention Russian’s responsibility for a military nerve agent attack on innocent people in my constituency is nauseating.”

READ MORE: Crisis looms as Holyrood set to pass its own alternative Brexit Bill

He added: “The European Commission President is appeasing a man, who poses a clear threat to Western security.”

The Foreign Office has called on the Russian Government to comply with its international obligations after a human rights watchdog found "restrictions on fundamental freedoms" had resulted in a lack of genuine competition in the country's presidential election.

The Organisation for Security and Co-operation in Europe issued an initial report by election monitors of Mr Putin's overwhelming victory that saw him returned to office with nearly 77 per cent of the vote. No 10 said it would give its response to the Russian election once the watchdog produced its full report.

Meanwhile, Jeremy Corbyn said he would still do business with Mr Putin despite "all fingers" pointing towards Russia being responsible for the Salisbury spy incident.

The Labour leader 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.”

READ MORE: Crisis looms as Holyrood set to pass its own alternative Brexit Bill

He went on: "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 cathedral city.