Simon Hughes has won the contest to be deputy leader of the Liberal Democrats.
The Bermondsey and Old Southwark MP saw off a challenge from Tim Farron, taking the votes of 38 of the party's 57 MPs.
Mr Hughes replaces Vince Cable, who stood down from the deputy leadership role last month to concentrate on the job of business secretary.
He said he would ensure the Lib Dems "seize the moment" provided by the coalition government deal with the Conservative Party.
An MP since 1983, Mr Hughes has served as the Lib Dems' president and was energy and climate change spokesman before the general election.
'Honour and privilege'
Neither he nor Mr Farron, who took 18 votes in the deputy leadership contest, was given a job in the coalition government
Both have suggested the Lib Dems should have "shadow" ministers for Whitehall departments where their parliamentarians are not represented.
After his victory was announced Mr Hughes said he was "fully supportive" of the agreement with the Tories, adding: "It is an honour and a privilege for me to follow Vince Cable as the new deputy leader of the Liberal Democrats. This is not a responsibility I will ever take lightly.
"I will fight every day for the principles which underpin our party: fairness, freedom, openness, equality, stewardship of our environment and standing up for those who cannot stand up for themselves.
"I will work tirelessly with Nick Clegg and all other Liberal Democrats to help liberal democracy blossom and flourish throughout the land."
Party leader Mr Clegg said: "I am delighted Simon has been elected as deputy leader.
"Simon has been a huge figure in the Liberal Democrats for decades. He's a tireless campaigner, a relentless fighter for the vulnerable and marginalised in our society, and one of the hardest-working MPs Parliament has ever seen.
"There are huge challenges and opportunities ahead for our country and our party, but with Simon by my side I am sure that we can meet those challenges and work to build a fairer, more equal Britain."
One Lib Dem MP abstained from the deputy leadership vote.
Mr Cable resigned last month saying it was right to "focus wholeheartedly" on his new job as business secretary.
Mr Hughes ran unsuccessfully for the party leadership in 1999 and 2006.