Rishi Sunak faces a fresh split among Tories after a former justice secretary argued controversial post-Brexit legislation on Northern Ireland is no longer legally justified.

Sir Robert Buckland’s intervention over the Northern Ireland Protocol Bill on Tuesday came after Boris Johnson pressured the Prime Minister not to drop the proposed law.

The row comes as Mr Sunak closes in on a deal with the European Union to alleviate the issues in the nation created by the UK leaving the trading bloc.

Sir Robert, who served under Mr Johnson, previously backed the Bill that could empower the UK to unilaterally scrap parts of the protocol in what the EU says is defiance of international law.

But now, in an article published in The House magazine, he argues that the “Bill’s political utility has run its course” as the Government tries to get a new deal.

It is the proverbial dead letter

He argued that when the legislation was introduced to the Commons by Mr Johnson back in June last year it seemed the “stalemate” with Brussels “would never end”.

“In short, things have changed dramatically since mid-2022,” Sir Robert wrote.

“The Northern Ireland Protocol Bill has outlived its political usefulness and no longer has any legal justification. It is the proverbial dead letter.”

Sources close to Mr Johnson made it clear over the weekend that he believes it would be a “great mistake” to drop the legislation, which is currently stalled in the House of Lords.

Former Cabinet minister Jacob Rees-Mogg backed the call, saying the Northern Ireland Protocol Bill had the support of Mr Johnson, who he described as “the person who had a mandate from the British voters”.

Some have interpreted words from Home Secretary Suella Braverman as support for Mr Johnson’s plea.

The longstanding Eurosceptic described the Bill as “one of the biggest tools that we have in solving the problem on the Irish Sea”.

Downing Street stressed on Monday that the Northern Ireland Protocol Bill remains “important” in “the absence of a negotiated solution”.