Lord Michael Ashcroft has resigned from the House of Lords as he can no longer "contribute meaningfully".
He has donated £10m ($15m) to the Tories since the 1980s but withdrew the lifeline in 2013 amid speculation he doubted the party would secure a majority at the general election in May.
His Lord Ashcroft Polls service into marginal seats is seen as a barometer of how the election could unfold.
A statement on his website said he was stepping down to concentrate on his Lord Ashcroft Polling service as well as publishing interests.
"Earlier this year Baroness D'Souza, the Lord Speaker, said that any Member of the House of Lords who can 'no longer contribute meaningfully' should retire," his statement read.
"She added that since the House has close to 800 members, 'retirement at the right time should be seen as a condition of membership of the House of Lords – a duty as well as a right'.
"I agree with the Speaker, and have concluded that my other activities do not permit me to devote the time that membership of the Lords properly requires.
"Accordingly, I have today written to the Clerk of the Parliaments giving notice of my resignation from the House of Lords with immediate effect, pursuant to Section 1(1) of the House of Lords Reform Act 2014.
"I will continue my involvement in politics through Lord Ashcroft Polls and my political publishing interests: Conservative Home, Biteback Publishing and Dods."
His final written question to the House of Lords came on 4 March when he asked what the government's response was to a recent leaked memo concerning Ethiopia's "Destabilising Policy Against Eritrea".
Ashcroft, who grew up in tax haven Belize and registered his companies there, was made a life peer in 2000 on condition he moved to the UK.
However, 10 years later he had still not moved and was criticised for his "non-dom" status. MPs also accused him of flouting laws banning overseas contributions.
As an author, Ashcroft has published seven titles and his philanthropic projects have included founding the charities Crimestoppers and Action on Addiction.