London law firms are delaying the sanctions levelled against their Russian oligarch clients by threatening to challenge the measures announced by the UK Government, Liz Truss has told MPs.

The Foreign Secretary was briefing MPs on the situation in Ukraine on Friday afternoon, according to Labour MP for Exeter Ben Bradshaw.

Mr Bradshaw, who was in the briefing, told the PA news agency Ms Truss had been asked why the sanctions process had not been moving more quickly.

He said that she told MPs it was “because they had to be very careful that when they sanctioned somebody it was legally watertight because these oligarchs’ lawyers in London are very litigious, and she had already had several warning letters from them”.

Mr Bradshaw called for the law firms in question to be named in the first instance, but also potentially subject to sanctions themselves.

“(They should be) not only named and shamed, but any law firm or any British institution that works on behalf of any sanctioned Russian should themselves be subjected to the same sanctions,” he said.

He said there had been a suggestion in the briefing that the sanctions should be introduced via emergency primary legislation rather than secondary legislation so they “wouldn’t be vulnerable to legal challenge”.

He said that would be an avenue “which of course Labour would support”.

But lawyer David Allen Green said it was “twaddle” that any challenge could delay the sanctions.

He tweeted that he was a former Government and City lawyer and that “if government gets its act together, there is ‘nothing’ City law firms can do for their oligarch clients”.

Jessica Simor QC added that: “Law firms can’t hold it up. Only a court could.”

On Thursday, officials recognised that there was a process by which people subject to sanctions could challenge the measures, and that the Government was prepared to fight cases in court if necessary.

They acknowledged that oligarchs are highly-litigious and were likely to seek to defend their interests, and therefore sanctions have to be legally sound.

The Prime Minister’s official spokesman was asked on Thursday whether No 10 had a message to those representing the oligarchs.

He said: “I think it’d be the same to any company or individual that is looking to engage with the Russian regime. It’s important we distinguish between the Russian people and Putin and his regime.

“We believe that the actions that Russia are taking, which is an invasion in Europe, of a democratic country, are completely unacceptable and there should be no engagement with them considering they have taken these actions.

“Putin and his regime should be treated like pariahs on the world stage.”

Boris Johnson said on Friday that he was considering sanctioning more of Vladimir Putin’s “inner circle” after Ukraine demanded tougher measures and support fending off the Russian invasion.

So far, eight oligarchs have been hit by British sanctions plus individual sanctions have been issued against more than 100 individuals, entities and subsidiaries.