A book by Michael Ashcroft, a former deputy chairman of Johnson’s Conservative Party, serialised in a newspaper over the weekend alleged Carrie had influenced the prime minister’s decision making in a negative way.

“It is sexist … Going after Carrie Johnson is undignified, it is unfair and it is wrong,” Javid told Sky News.

Javid also said the allegations were misogynistic.

“As a general rule, a politician’s partner … should be off limits,” he told BBC Television.

Carrie, 33, a former head of communications for the Conservative Party and former senior adviser to ministers including Javid, is Johnson’s third wife and the couple have two young children.

Her spokesperson said the allegations in Ashcroft’s book were “a brutal briefing campaign against her by enemies of her husband”.

“This is just the latest attempt by bitter ex-officials to discredit her. She is a private individual who plays no role in government,” the spokesperson said.