Boris Johnson is seeking to cement the support of foreign ministers of the 27 other EU member states on the poisoning of Sergei and Yulia Skripal at a meeting to be held on Monday in Brussels, as the UK tries to rally more international support behind its tough stance against Moscow.

Senior government insiders say ministers hope that the Foreign Affairs Council would, as a minimum, issue a joint statement condemning the attack and hold initial discussions on possible further moves that could be taken at an EU level.

Since 2014 the EU has imposed a series of economic sanctions and other measures against Russia, including asset freezes and travel restrictions, over its illegal occupation of Crimea and destabilisation of Ukraine. Foreign Office sources said that it was too early to say if further EU action could be taken as a result of the Salisbury attack.

Sir Roderic Lyne, a former British ambassador to Russia, said Johnson’s task would inevitably be harder because the UK was leaving the EU. “The sanctions imposed by the EU after Crimea in 2014 surprised the Kremlin and have continued to have an impact, precisely because they remain EU-wide,” he said.

“Brexit has made Britain’s task harder in appealing for EU solidarity this week. The UK alone cannot have a big impact on Russia. The EU does, especially when it can work with the US and other Nato allies.”

On Saturday, after Moscow retaliated against the expulsion of 23 Russian diplomats from London by dismissing the same number of UK representatives, the prime minister, Theresa May, said she was encouraged by the level of international solidarity behind her response to the Salisbury nerve gas attack.

“We can be reassured by the strong support we have received from our friends and allies around the world,” she said. “From the United States, Nato and the European Union [and] from our UN and Commonwealth partners.”

The Foreign Office issued a statement saying the next moves would be considered by senior intelligence, military and Whitehall officials this week: “In light of Russia’s previous behaviour, we anticipated a response of this kind and the National Security Council will meet early next week to consider next steps. Our priority today is looking after our staff in Russia and assisting those that will return to the UK.”

Labour MPs will step up pressure on the government to introduce tougher measures against wealthy Russians investing in the UK. The shadow chancellor, John McDonnell, is proposing a series of new measures, including an “oligarch tax” – a charge against purchases of residential property by offshore trusts located in tax havens.

McDonnell said: “If we want to really take the fight to the gangster politicians and Russian elites hiding their money in the UK, then we need serious measures which will hit them where it hurts – in their wallets. It’s time to call an end to the use of our financial system and property market as a hiding place for foreign oligarchs and their money men by implementing measures like full transparency for tax havens and our levy on secret offshore property purchases.”

Labour is also demanding a tightening of the “politically exposed persons” regime, which is intended to provide additional scrutiny by financial institutions of those judged likely to indulge in corrupt practices. At present the definition applies to high-level government figures, their families and their close associates. McDonnell wants to widen the definition to catch more in the group who are labelled associates.

After Jeremy Corbyn was criticised by MPs in his party for what many saw as an insufficiently strong line against Moscow in the House of Commons last week, an Opinium/Observer poll shows the public back Theresa May over the Labour leader to best handle the current crisis by a margin of more than two to one. Asked which of the two leaders would be best at reacting to Russia’s likely involvement in the nerve gas attack, 39% said May, while only 16% selected Corbyn.