British politician Michael Ashcroft accuses Johnson's wife of preventing him from leading effectively

In his biography, British politician Michael Ashcroft has accused British Prime Minister Boris Johnson's wife, Carrie of preventing him from leading effectively.

"My book shows Carrie's behaviour is preventing the prime minister leading Britain as effectively as the voters deserve," wrote Ashcroft, a former deputy chairman of the Conservatives.

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British health minister Sajid Javid said, "It is sexist ... Going after Carrie Johnson is undignified, it is unfair and it is wrong."

Javid also said the allegations were misogynistic.

"As a general rule, a politician's partner ... should be off limits," he told BBC Television.

After the first installment of Ashcroft's book was published in the Mail newspaper, her spokeswoman dismissed the claims as part of a smear campaign against the couple by "bitter ex-officials to discredit her."

Carrie Johnson, she said, was a "private individual who plays no role in government".

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.

She has given no interviews, and only appeared by Johnson's side at marquee events such as international summits and last October's annual Tory conference.

But she has increasingly been accused of holding too much sway over issues close to her heart -- especially the environment and animal rights -- and over staff appointments.

Much of the offensive has come from the prime minister's embittered former chief aide, Dominic Cummings.

Cummings has claimed she wanted to "get rid" of him from Number 10, and alleged she tried "to change a whole bunch of different appointments".

(With inputs from agencies)