Lord Rennard helped the Lib Dems to a record performance in 2005 election
Lib Dem chief executive Lord Rennard is to resign, a spokeswoman has announced.
She said he was standing down for health and family reasons and had been discussing it for some time with the party leader Nick Clegg.
She said it was not linked to criticism of his expenses. He is reported to have claimed over £40,000 for a second home when he owned a flat near Westminster.
Lord Rennard said at the time that the basis of his claims were approved by the House of Lords authorities.
The peer, the party's chief election campaign strategist, will step down from his position at the end of the summer.
However, he will continue to run the party's campaign for local council and European elections next month.
Responding to the announcement, Mr Clegg said Lord Rennard would be "sorely missed".
"It is impossible to exaggerate Chris's immense contribution to the Liberal Democrats over the years.
"Without Chris's unique skills as one of the country's most astute and effective political campaigners, I doubt that the party would now have the largest number of MPs in decades."
He said he had steered the party "through some turbulent times" and had made its organisation more professional.
"At all times, he has been utterly loyal, hard working and dedicated to the wider good of the party," he said.
In a statement announcing his planned departure, Lord Rennard said he wanted more time "outside the Westminster bubble" for himself and his family.
He said he would continue to support Mr Clegg and the party but no longer in a day-to-day capacity.
"I want to be able to work more flexibly in future whilst of course continuing to help our party advance," he said.
"I believe that I will be better able to do so without the administrative burdens of being chief executive and running the party's day-to-day organisation."
Lord Rennard, who was made a life peer in 1999, is credited with masterminding the party's electoral advance in recent years - firstly as director of campaigns and then as chief executive.
He helped pull off a series of spectacular by-election victories over the past 20 years.
The party won 62 seats in the 2005 general election, its best performance since the 1920s.
However, the party has enjoyed mixed fortunes since then with both Charles Kennedy and Sir Menzies Campbell being forced out as leaders.