Tory peer Lord Ashcroft has relinquished his seat in the House of Lords to concentrate on other pursuits.
The former party treasurer said he was resigning with "immediate effect" as his other activities meant he did not have the time to devote to his duties in the upper chamber.
"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," he said in a statement posted on his website.
"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."
Lord Ashcroft, who remains a life peer, said he intended to continue his involvement through his polling organisation and his publishing interests.
