Ms Harman, the Labour MP for Camberwell and Peckham, was almost 30mph above the 70mph limit when she was stopped on the M4 in Wiltshire while taking her son back to university in Bristol after the Christmas break.

A spokeswoman for the Attorney-General’s Office said that she intended to plead guiltyat the first available opportunity. “Harriet Harman is very sorry that she exceeded the speed limit. She fully recognises that she should have paid more attention to the speed at which she was travelling,” she said. The spokeswoman said that police had not yet told Ms Harman about a possible date for a court appearance.

As Solicitor-General, Ms Harman, 52, the first woman to hold the post, is one of the most senior law officers in the country.

A source close to her said last night that she was mortified about the incident. “She does not intend to offer any excuses or argue mitigating circumstances in any way. She has thrown her hands up.

“It is the first speeding offence she has had in her life and she is understandably very angry with herself. There is nothing she can say other than how sorry she is.”

Ms Harman will consider herself relatively lucky to be caught speeding at 99mph. Drivers exceeding 100mph can usually expect a driving ban of at least a month and a fine of around £800.

However, for a first offence, she can expect to have her driving licence endorsed with a number of penalty points and a fine. She is also likely to see her motor insurance premium increase when it is renewed.

The incident follows one in 2000 when the chauffeur to Jack Straw, then Home Secretary, was stopped by police driving at 103mph along the M5 near Taunton. No fixed penalty was issued.

A few months later, when it was decided that the chauffeur would not be charged, it was claimed that he had accelerated to avoid a potential risk after spotting another car approaching at high speed.

John Prescott, the Deputy Prime Minister, has four speeding convictions, the last in 1997 when he drove at 80mph on a 30mph section of the icy M62 motorway. He said that he was going to a constituency surgery and did not want to keep people waiting in the cold.

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