LONDON (AP) — A senior member of Prime Minister Boris Johnson’s Cabinet resigned Saturday night, adding to a sense of disarray within a government that has faced rebellion from his own lawmakers and voters this week.

Brexit Minister David Frost said in a letter to Johnson that he was stepping down immediately after a newspaper reported that he had planned to leave the post next month.

Frost said the process of leaving the European Union (EU) would be a long-term job. “That is why we agreed earlier this month that I would move on in January and hand over the baton to others to manage our future relationship with the EU,″ he said in his resignation letter.

However, the Mail on Sunday said earlier that he resigned because of growing disillusionment with Johnson’s policies. The newspaper said Frost’s decision was triggered by last week’s introduction of new pandemic restrictions, including a requirement that people show proof of vaccination or a negative coronavirus test to enter nightclubs and other crowded venues.

And in his resignation letter, Frost said the UK needed to “learn to live with Covid.”

The news follows a stunning defeat for Johnson’s Conservative Party in a by-election Thursday in North Shropshire, a long-time party stronghold. Earlier this week, 99 Conservative lawmakers voted against so-called vaccine passports in the House of Commons, the biggest rebellion in Johnson’s 2 1/2 years as prime minister.

Frost’s resignation comes after the UK recently softened its stance in the talks with the EU over post-Brexit trade rules for Northern Ireland. The change of tone from Britain came as a surprise to many because it seemed at odds with the hardline position of the Brexit minister, who was nicknamed “Frosty the No Man.”

Johnson’s government is also under fire over reports that officials held Christmas parties last year when pandemic rules barred such gatherings.

Adding to his problems with the so-called partygate scandal, Johnson’s choice to investigate the claims had to step aside after he also was tied to such parties.

Simon Case, the head of the civil service, stepped aside from the investigation after the Guido Fawkes website reported Friday that his department held two parties in December 2020.

The scandal erupted when a video surfaced showing a mock news conference at which some of Johnson’s staff appeared to make light of a party that violated the pandemic rules. Until that time, the prime minister had steadfastly denied government officials had broken any lockdown rules.