Britain's Prime Minister Boris Johnson (second from left) attends the Remembrance Sunday ceremony at the Cenotaph on Whitehall in central London, on November 14, 2021. Remembrance Sunday is an annual commemoration held on the closest Sunday to Armistice Day, November 11, the anniversary of the end of the World War I and services across Commonwealth countries remember servicemen and women who have fallen in the line of duty since the war.

Prime Minister Boris Johnson has thrust Britain into a constitutional crisis by breaking the law he set for pandemic restrictions, effectively "shredding the ministerial code," the country's leading constitutional expert said on Sunday.

Peter Hennessy, a historian and member of the upper house of parliament, said Johnson had become "the great debaser in modern times of decency in public and political life" after he was fined by police for attending a social gathering in Downing Street while lockdown restrictions were in place.

The ministerial code sets out the standards of conduct expected of ministers and how they discharge their duties, according to the government website.

Johnson has been accused of misleading parliament over the matter by opposition lawmakers after he told parliament in 2021 that all rules were followed in Downing Street during the pandemic. He will appear in the House of Commons on Tuesday to explain why he was fined by police.

He has also apologized after he became the first British leader found to have broken the law while in office. Police are investigating further gatherings and he could receive further fines.

"I think we're in the most severe constitutional crisis involving a prime minister that I can remember," Hennessy told BBC Radio, asking why anyone in public life would adhere to the rules when the prime minister did not.

"The prime minister sealed his place in British history as the first lawbreaker to have occupied the premiership."

Jacob Rees-Mogg, a minister in Johnson's cabinet, said he respected Hennessy but did not think the constitutional expert had fully understood the constitutional significance of the ministerial code.

Reuters