The former Tory treasurer said he will give at least half of his $1.8 billion fortune to charity as he joins a movement led by Warren Buffett and Bill Gates, America's two richest men, The Sunday Times reported.
Another six of Britain's rich have already made the pledge.
Ashcroft's commitment comes as the 25th anniversary of The Sunday Times Rich List indicated that the richest 1,000 people in Britain have a total of $685 billion in assets — up $53 billion from last year.
Britain's wealthiest person was named as Alisher Usmanov, the Russian businessman who owns a third of the Premier League football club, Arsenal, The Guardian reported.
Usmanov has $20 billion in assets.
Lakshmi Mittal, who spent eight years at the top of the list, has dropped to fourth place in this year's survey.