Theresa May has come under pressure to plan strong retaliation against Russia over the poisoning of former spy Sergei Skripal, ahead of security talks with the Cabinet and after a fresh warning to people in Salisbury.

MPs and the widow of murdered dissident Alexander Litvinenko both demanded a tougher stance against Vladimir Putin, as the finger of suspicion continued to point to a Kremlin plot.

A meeting of the National Security Council (NSC) on Monday is expected to see senior ministers also call for an agreement to beef up Britain’s response.

In the Wiltshire city, hundreds of diners and pub-goers were told to wash their clothing and possessions, after traces of the nerve agent used to poison the double agent and his daughter were found.

Anyone who was in The Mill pub between 1.30pm last Sunday and 11.10pm on Monday, or the nearby Zizzi restaurant between 1.30pm on Sunday and 9pm on Monday, was urged to follow the advice.

Senior ministers, led by Foreign Secretary Boris Johnson, are expected to use the special meeting of the NSC to urge stronger measures, despite no confirmation that Russia was responsible for the nerve-agent attack.

Mr Johnson accused Russia of “acts of war” in an inflammatory Commons statement last week, to the fury of its leaders who have denied any involvement in the attack on the country’s former intelligence officer.

The Prime Minister, who chairs the NSC, is reported to have silenced Mr Johnson in Cabinet last Tuesday when he said Russia was responsible – but he is thought to have Gavin Williamson, the Defence Secretary, among his allies.

They are likely to be emboldened by criticism of British timidity by the widow of murdered Russian dissident Alexander Litvinenko, who accused Ms May of broken promises.

Marina Litvinenko produced a letter, sent to her by Ms May, when Home Secretary, promising to “continue to pursue justice”, to ensure no repeat of her husband’s killing on British soil.

“Unfortunately, it happened again and the lesson after the murder of my husband was not learned,” she told the BBC’s Andrew Marr Show.

Ms Litvinenko called for the return of more than £820,000 donated to the Tory party by Russian oligarchs, since Ms May came to power, saying: “You need to be very careful who you are friends with.”

But Philip Hammond, the Chancellor, refused the plea – insisting the cash was legitimate and denying the Kremlin was “laughing at us” over Britain’s response to the Litvinenko killing and the Salisbury poisoning.

He will attend the NSC meeting and warned: “If there were to be an involvement of a foreign state, evidenced by this investigation, then obviously that would be very serious indeed and the government would respond appropriately.”

Meanwhile, Labour and Tory rebels accused the Government of dragging its heels on a British version of the US “Magnitsky Act”, to target foreigners involved in human rights abuses.

The 2012 American legislation was designed to punish Russian officials involved in the suspicious death in a Russian jail of the lawyer Sergei Magnitsky while he was investigating official corruption.

Labour claimed the Government halted an expected three-hour committee last month after just 25 minutes, fearing a vote on amending sanctions legislation would be lost.

Conservative MPs seeking to insert “Magnitsky amendments” into the Sanctions and Anti-Money Laundering Bill said ministers were resisting a key element necessary to ensure the law was actually used.

In Salisbury, the authorities were forced to defend issuing the new guidance to residents, one week after Mr Skripal, 66, and his daughter, Yulia, 33, were found on a bench in the city.

Both remain in Salisbury District Hospital in a critical, but stable condition, as efforts continue to establish beyond reasonable doubt who carried out the devastating nerve agent attack.

Dr Jenny Harries, joint director of Public Health England, told a press conference: “This is about a very, very small risk of repetitive contact with traces of contamination that people may have taken out.

“The advice we’re giving today about washing clothes – very simple things ... that will remove that risk as we go forward.”

Up to 500 people in either The Mill pub or the Zizzi restaurant, between Sunday lunchtime and Monday night, were also urged to double-bag clothing earmarked for dry cleaning and wipe personal effects such as mobile phones, purses and wallets with baby wipes.

Wiltshire Police also told residents “not to be alarmed” after the military removed a number of vehicles and objects from in and around Salisbury.

Troops, including Royal Marines, were supported by firefighters, police and medics in the now-familiar hazmat suits and gas masks, in the biggest military operation yet.

A military forklift truck was used to lift cars onto the back of low-loader trucks, where they were covered before the military removed them from the scene.