A key ally of former British Prime Minister Boris Johnson formally resigned Saturday, delivering a blistering attack on his successor in her resignation letter.

Former culture minister Nadine Dorries announced her intention to resign 11 weeks ago but had remained as member of parliament while she investigated why she was not given a seat in the upper house of parliament.

In her resignation letter, Dorries accused Prime Minister Rishi Sunak of abandoning “the fundamental principles of Conservatism" and said "history will not judge you kindly".

Dorries was unexpectedly not awarded a seat in the upper chamber of parliament, the House of Lords, in Johnson's resignation honours list. He rewarded fellow Brexit die-hards and even those implicated in the "Partygate" scandal that contributed to his downfall last year.

The omission prompted accusations from Johnson's camp of meddling from Sunak and Downing Street.

In her letter, which she released on social media, Dorries accused Sunak of leading attacks on her resulting in "the police having to visit my home and contact me on a number of occasions due to threats to my person.

"The clearly orchestrated and almost daily personal attacks demonstrate the pitifully low level your government has descended to," she wrote.

She also attacked his record in Government.

"Since you took office a year ago, the country is run by a zombie parliament where nothing meaningful has happened," she wrote.

“You have no mandate from the people and the Government is adrift."

Resignation honours lists, in which key political allies of outgoing prime ministers are rewarded, are normally waved through by their successor.

- Political revenge -

Sunak's spokesman told reporters earlier it was "entirely untrue" that the prime minister or officials removed names from Johnson's list before it was sent to a House of Lords vetting committee.

The row in June over the honours prompted the resignation as MPs of both Nigel Adams, who was also omitted from the list, as well as Johnson himself.

At the same time Dorries announced her intention to resign.

Political observers have interpreted the resignations as Johnson's revenge on Sunak for forcing him out of office last July after "Partygate" and a string of other scandals.

Johnson resigned as an MP claiming a stitch-up by political opponents on a cross-party inquiry probing whether he lied to parliament over the Covid lockdown-breaking parties at Downing Street.

The resignations meant Sunak's Tories had to face by-elections at a time when they were trailing in the polls to the main opposition Labour party and ahead of a general election next year.

The Conservatives lost Adams' seat but held on to Johnson's constituency, aided by Labour mayor Sadiq Khan's contentious expansion of a vehicle pollution tax to outer London.

The government now faces the prospect of another by-election, possibly within weeks, although Dorries' 24,000 majority in her constituency might prove more difficult for the Labour opposition party to overturn than Adams' 20,000 majority.