Alastair Campbell has claimed "senior" figures in Jeremy Corbyn's office recommended voting against Labour, after he was expelled from the party for backing the Liberal Democrats in the European elections.
Tony Blair's former spin doctor, a leading campaigner for a second EU referendum, said the Labour Party faces "oblivion" unless it clarifies its policy on Brexit.
Speaking outside his north London home on Tuesday, Mr Campbell also told reporters he will appeal against Labour's decision to expel him.
"I think that there are people in Jeremy Corbyn's office, senior positions in Jeremy Corbyn's office, who have recommended voting against the Labour Party," he said.
"You can interpret the rules in all sorts of different ways, but one thing I know is I'm not going to leave the party just because some random email comes in telling me that I've been expelled.
"So I will definitely appeal against it and we will see where that goes."
He described his rapid expulsion as "strange" and added: "I think people will inevitably draw the contrast with the lack of rapidity in dealing with cases involving anti-Semitism."
Labour has said "support for another political party or candidate is incompatible with party membership".
But Mr Campbell won support among other Labour grandees, such as ex-home secretary Charles Clarke, who called for him to be reinstated.
Mr Clarke, who said he had been a Labour member for 47 years, said he also voted Lib Dem in the European elections as a "one-off" over his party's "hopeless incoherence" on Brexit.
Labour and the Conservatives received a drubbing in the European elections as Nigel Farage's Brexit Party romped to victory and the Liberal Democrats took the second largest share of the vote.
Mr Campbell said Mr Blair, who was Labour prime minister between 1997 and 2007, refused to withdraw the Labour whip from Mr Corbyn when he rebelled.
"Tony was of the view, no, you've got to have dissenting voices around," he said.
"Jeremy Corbyn himself, for reasons best known to himself, has in the past congratulated George Galloway on defeating a Labour candidate."
He said he is "still in the Labour Party" and will "always be Labour".
Mr Campbell, a key player in the New Labour era, outlined his disappointment at his expulsion in a series of tweets.
He added: "I know there are an awful lot of MPs and councillors and peers who did not vote for Labour in the European elections and they did it for the same reasons I did it, to try and get the Labour Party to see sense on the single most important issue facing the country."
Mr Campbell declined to say who he would vote for in a snap general election, adding: "I want to vote Labour at the general election but that will depend on the policy that the Labour Party puts forward between now and then in relation to Brexit."
Labour rules say members who support a party other than Labour are automatically ineligible for membership.
Mr Campbell's expulsion was described as an "absurd act of political self-harm" by former Labour MP Andrew Mackinlay, who announced he had joined the Liberal Democrats on Tuesday morning.
"I have let my Labour Party membership lapse," he said, adding: "The Liberal Democrats' call for a People's Vote is, unlike the Labour Party, consistent and unambiguous."