Deputy Chief Whip John Randall has resigned from the Government, Downing Street said ahead of an expected ministerial reshuffle by David Cameron.
The 58-year-old Conservative MP for Uxbridge and South Ruislip said in his resignation letter that he had indicated at the end of last year that he wished to step aside.
In his resignation letter to the Prime Minister, he said it had been " a great privilege and honour" to serve for 13 years in the Whips' Office in opposition and Government.
"I have nothing but the deepest admiration for you as a person, leader and Prime Minister. I will never forget the kind note that you wrote to me when my mother died last year," he said.
"You can be assured that I will do whatever I can for you personally as well as for the party, the Government and of course the country."
The Prime Minister said he "could not have wished for a more loyal, discreet, patient, trustworthy and committed colleague" and that he " had rather hoped this day would never come".
"You have been a rock, not just in the Whips Office since 2000 where you have served with great distinction as Assistant and then Deputy Chief Whip, but for the whole Parliamentary Party.
"Your wit and humour are well known across the Party, but so too is your compassion; your dedication to the Party and to Parliament; and your steadfast reliability in good times and bad."