Alastair Campbell has been expelled from the Labour Party after voting Lib Dem in the European elections.
The former chief spin doctor to Tony Blair said he was "sad and disappointed" as he faces a five-year wait before he can rejoin the party.
Mr Campbell is a vehement critic of Jeremy Corbyn's Brexit stance and wants a second referendum to keep Britain in the EU.
On Sunday night he announced on the BBC: "I voted Liberal Democrat. I didn’t vote Labour - for the first time in my life - and it was a very strange feeling.
“I just felt on this issue at this time the Labour Party has let its own supporters down, it’s let its members down and it’s let the country down.”
He vowed today to launch an appeal.
And he issued a blistering attack on party chiefs by comparing his swift expulsion with the speed of action against Labour members accused of anti-Semitism.
Labour sources confirmed Mr Campbell was "ineligible" for membership.
It is understood he underwent a process known as "auto-exclusion" rather than being formally expelled, which requires a disciplinary panel.
Under this process he cannot reapply for Labour Party membership for five years unless party chiefs agree to hear his case earlier, Labour sources said.
He could reapply after two years if there are "exceptional circumstances".
Sources added is no internal appeal process for auto-exclusion - only a 14-day window in which Mr Campbell can provide evidence that he was not ineligible for membership.
Yet because he confessed backing a rival party on national TV, this appears unlikely to succeed.
That could force Mr Campbell to seek redress in the High Court - which ruled on challenges to Labour Party rules after Jeremy Corbyn took power.
A Labour spokeswoman said: "Support for another political party or candidate is incompatible with Party membership.
“The Lib Dems cannot and will not end austerity. They cannot bring our country together or be trusted to deliver on their promises.
"They propped up the Tories for five years and imposed the austerity that has devastated our communities.
"Labour will do things very differently, and ensure our society is run for the benefit of the many, not just a privileged few."
Mr Campbell announced the news on Twitter, where he wrote: "Sad and disappointed to receive email expelling me from Labour.
"Particularly on a day leadership finally seems to be moving to the right place on Brexit, not least thanks to tactical voting by party members, including MPs, councillors and peers who back People's Vote.
"I was not intending to publicise this at this stage.
"But [I] have had calls from friends in the Party telling me it is now widely known and likely to be leaked.
"I have been advised by lawyers with expertise in this field I have grounds for appeal against expulsion and shall do so.
"I am and always will be Labour. I voted Lib Dem, without advance publicity, to try to persuade Labour to do right thing for country/party.
"In light of appeal, I won't be doing media on this.
"But hard not to point out difference in the way anti-Semitism cases have been handled.
"Plenty of precedent of members voting for other parties/causes. Some are now senior Party staff.
"Approach also contrasts with our era when Tony Blair was being pressed by whips to withdraw whip for Jeremy Corbyn and others for persistently voting against Labour in Parliament, and he said No."
This breaking news story is being updated.
