Chuka Umunna said he is the opposite of a “careerist” despite joining his third party in six months.
The former Labour and Change UK MP has defected to the Liberal Democrats, a party he once accused of “betraying students” over tuition fees.
He said: "I get called a careerist but... the things I've done are the antithesis of what it is to climb up the greasy pole."
“You don't leave the political security of one of the two main parties to further your career - anyone can tell you that."
He added: “The easy thing is to sit in your comfort zone in a party that you know is doing the wrong things, that has changed beyond belief.”
The move to the Liberal Democrats has attracted criticism after old tweets emerged where Mr Umunna said “you can’t trust a word the Lib Dems say”.
Tweets from Mr Umunna slamming the Liberal Democrats have reemerged
The Streatham MP said the party has changed its stance on austerity, which he criticised heavily during his time as Labour’s shadow business secretary.
He has declined to back a candidate in the leadership race between Sir Ed Davey and Jo Swinson.
“I'm very conscious I've only just arrived in the Lib Dems and I have to earn the trust and gain the confidence of its membership,” he said.
“If I just walked in and said right, I think that person should be leader now, that would be rather arrogant and presumptuous.”