Boris Johnson was under pressure to go faster in targeting Russian oligarchs and seizing their assets in Britain, after it was conceded it could take “weeks and months” to build legally-sound cases.

Downing Street said on Thursday it would consider possible changes to speed up the process as ministers sought to apply pressure on Russian president Vladimir Putin over his invasion of Ukraine.

Senior Tories called for the immediate seizure of oligarchs’ assets in the UK, such as luxury yachts and property, and the return of them to the Russian people “as soon as possible”.

Questions continued over why billionaires such as Roman Abramovich, who has announced he will sell Chelsea FC, have not been hit with sanctions.

A Government source acknowledged to the PA news agency it could take “weeks and months” to build a legally watertight case against some Russian oligarchs.

“We’re working round the clock and going as quick as we can,” the source added, with Foreign Secretary Liz Truss understood to have tripled the size of the sanctions team in recent months.

Visiting Lithuania to show support to Nato allies, Ms Truss acknowledged in an interview that the Government must “make sure we have the right evidence to put in place those sanctions”.

But she added: “I’m very clear that legal threats will have no impact on our ability to sanction oligarchs and we will continue to work through our list, we will continue to sanction oligarchs and there is nowhere for any of Putin’s cronies to hide.”

Downing Street sought to downplay the issue, with the Prime Minister’s official spokesman insisting: “We are not being held back from introducing sanctions.”

But he said “we do have laws that we need to abide by” when applying the economic restrictions.

“When it comes to individuals it is the case that we need to do the preparatory work, the requisite work, to make sure it is legally sound before introduction,” the spokesman added.

“Like I said, we will keep that under review and if there are ways to further speed it up then we will.”

We should be looking immediately to seize those assets linked to those who are profiting from Putin’s war machine, holding it in trust and returning it to the Russian people as soon as possible

He also sought to argue that sanctions on the banks funding the Russian president’s military machine will exert more pressure than going after his wealthy allies.

“Our judgment is placing sanctions particularly on large banks and companies … that is what we believe will exert the most pressure on Putin’s regime and will throttle off funding for this illegal war against Ukraine,” he said.

But Tom Tugendhat, the chairman of the Commons Foreign Affairs Committee, called for the Government to go further to follow European allies to seize oligarchs’ assets.

“We should be looking immediately to seize those assets linked to those who are profiting from Putin’s war machine, holding it in trust and returning it to the Russian people as soon as possible,” he told PA.

Senior Tory MP Tobias Ellwood echoed the call, warning there will be “increasing public anger that we’re not doing enough to help our fellow Ukrainians in their hour of need”.

The chairman of the Commons Defence Committee told PA: “There’s a race to squeeze Putin given the war crimes he’s now committing in Ukraine and London continues to be seen as ground zero as to where oligarchs’ investments sit. So we need to be impounding these assets in days, not weeks or months.

“Every day we wait offers more time for the oligarchs to move their wealth to other parts of the world. Don’t forget it’s not their wealth, this is the stolen wealth from the Russian people which is utilised to keep Putin in power.”

French authorities have said they seized a yacht linked to Igor Sechin, an ally of Mr Putin who runs oil giant Rosneft, under EU sanctions. It was also reported Germany had seized another megayacht.

It was understood UK officials had been tasked with looking at legal routes for seizing the luxury British properties of oligarchs with ties to Mr Putin without paying them compensation.

“It is time to shut down the racket of illicit money in British property,” a Government source said.

Mr Abramovich, the Russian-Israeli billionaire who has owned Chelsea since 2003, announced he would sell the club, with the “net proceeds” going to a charity he would set up to “benefit of all victims of the war in Ukraine”.

His statement, which avoided any criticism of Mr Putin, came after politicians including Labour leader Sir Keir Starmer called for Mr Abramovich to face sanctions.

Labour’s shadow foreign secretary David Lammy called for new legislation to be brought to the House of Commons early next week so the UK’s sanctions regime on individuals “can catch up with our allies and partners”.

“It is totally unacceptable that a week after Putin’s illegal invasion of Ukraine just 11 oligarchs have faced sanctions by the UK Government,” he said.

Ukrainian presidential adviser Mykhailo Podolyak said a second round of talks was unable to broker peace but he did offer hopes of “humanitarian corridors” to allow civilians to flee areas of fighting, a suggested echoed by Mr Putin.

The port city of Kherson became the first major city to fall since the invasion began a week ago, with Russian military claiming to have seized control and Ukrainian officials saying that forces have taken over local government headquarters.

The Kremlin was pressing its offensive on multiple fronts, but a long column of tanks has apparently been stalled outside the capital of Kyiv for days.

Speaking during a visit to Estonia, Defence Secretary Ben Wallace said a “line has clearly been crossed” by Moscow by deploying thermobaric “vacuum bombs” which could indiscriminately kill civilians if used in Ukrainian cities.

It was understood that Mr Wallace was not suggesting that vacuum bombs, which suck in oxygen to create a devastating, high-temperature blast, have been used in Ukraine.

His comments came after the International Criminal Court (ICC) opened a war crimes investigation on Wednesday night after Britain and 37 allies referred Moscow over what the Prime Minister described as “abhorrent” attacks.

Ukraine has said that more than 2,000 civilians have died during the assault, as a humanitarian crisis unfurled in Europe as the United Nations said more than one million people have now fled to seek sanctuary from the Russian invasion.