BORIS Johnson was under pressure to go faster in targeting Russian oligarchs, 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 Tory MP Tom Tugendhat called for the immediate seizure of assets such as luxury yachts and property and return of them to the Russian people “as soon as possible”.

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

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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.

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

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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.”

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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 Tugendhat, the chair 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.