Lord Sugar has said he is quitting the Labour Party after 18 years over its "negative" stance on business.
The star of TV show The Apprentice, who is among Labour's largest donors, said on Friday the party had been "aware of my disillusionment for some time".
Lord Sugar said he kept his intentions secret during the election campaign in order not to damage Labour's chances.
He plans to stay in the House of Lords, having been granted a life peerage under Gordon Brown in 2009.
Lord Sugar served as an "enterprise champion" under Mr Brown's government and was among Labour's most prominent supporters.
He is also one of the party's largest donors - giving it £163,827 after Ed Miliband became leader in 2010.
But in a statement, the businessman said he had lost confidence in the party over the past year and had expressed concern over Labour's direction to its "most senior figures".
He said: "In the past year I found myself losing confidence in the party due to their negative business policies and general anti-enterprise concepts they were considering if they were elected. I expressed this to the most senior figures in the party several times.
"I signed on to New Labour in 1997 but more recently, particularly in relation to business, I sensed a policy shift moving back towards what Old Labour stood for.
"By the start of this year I had made my decision to resign from the party whatever the outcome of the general election."
'Not sticking boot in'
Lord Sugar added that he was a "loyal person" and decided to keep his intentions quiet during the election campaign.
"I have no wish to stick the boot into the party. There are many good people in Labour working hard every day to serve the public and I wish them all the best of luck. I am grateful for all the experiences being a member of Labour has brought me.
"I intend to continue in the House of Lords, representing the interests of business and enterprise in the UK. I will be making no further comment at this time."
Ben Bradshaw, the former Labour culture minister, urged him not to quit.
In a tweet, Mr Bradshaw said: "This is just the time *not* to leave but to stay & help us back to sanity ensuring we win in 2020."
Lord Sugar's announcement follows criticism of Labour strategy from other prominent party figures, including ex-PM Tony Blair and former minister Peter Mandelson.
Writing in the Observer newspaper on Sunday, Mr Blair said Labour needed to "appeal to those running businesses as well as those working in them".