Harriet Harman, the Solicitor General, was stopped by police yesterday for speeding on the M4 in Wiltshire.
The incident, involving one of the country's most senior law officers, is likely to embarrass the Government.
Ms Harman QC, the MP for Camberwell and Peckham, was driving her son to university in Bristol when she was stopped for allegedly travelling at almost 30mph above the speed limit.
A Downing Street spokesman said: "Harriet Harman said she is sorry and made clear she intends to plead guilty."
A spokesman for the Attorney General's Office said: "Ms Harman's speed when stopped was recorded as 99mph in a 70mph zone.
"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 intends to plead guilty to the speeding offence at the first available opportunity."
Ms Harman, an MP since 1982 and Secretary of State for Social Security from 1997-98, was appointed to the post of Solicitor General in 2001 after a spell on the back benches. She had been dropped from the Cabinet in Tony Blair's first reshuffle the year after the 1997 Labour landslide.