Jeremy Corbyn's deputy has claimed the party's future has been put at risk by the prospect of a hard Left takeover.
Tom Watson said there was a real threat to Labour's existence from the Corbyn supporters' group Momentum seeking to get union backing.
A fresh round of infighting was underway on Monday after Mr Watson said he was "deeply shocked" by a recording of Momentum leader Jon Lansman discussing how the country's biggest union, led by Len McCluskey, could affiliate with his group.
In the leaked recording, Mr Lansman issues a call to arms to Momentum supporters saying they need to make sure the Left is represented in key positions at all levels in the party.
Supporters of Mr Corbyn are said to be seeking influence within key committees in the party, in order to secure a change in party rules which would help a left-wing MP to succeed him as leader.
Mr Lansman is said to express confidence, at the meeting on 1 March, that the Unite union and Communication Workers Union (CWU) will join forces with his campaign group if Mr McCluskey is re-elected as general secretary in a contest next month.
Mr Watson said on Monday it would be "unprecedented" for Momentum to secure such political influence and access to members' money to organise within the Labour Party.
The deputy leader told Sky News: "It looks like a secret plan to take over the Labour party to me, and I think it needs to be called out.
"Many of our young members who have joined Momentum do not want to be involved in covert plans to take over the Labour Party.
"They want to be talking to the British people about the future our country has. There are powerful forces bearing down on this country.
"The threat to our union, potential uncertainty about the Brexit negotiations and we've just put troops in Estonia. We don't want to be distracted by this… we are facing a potential early General Election."
Mr Watson - who is joining the rest of the shadow cabinet on what promises to be a tense away day - added: "I'm not sure if Jeremy knows there's a secret plan.
"I'll be raising it with him today… I regard this as a battle for the future existence of the Labour Party. This is high stakes."
Unite boss Mr McCluskey is running for re-election as general secretary next month and Momentum is backing him, rather than moderate rival Gerald Coyne.
In a major split within Labour's top team, shadow chancellor John McDonnell, an ally of Mr Corbyn, claimed Mr Watson's attack was an attempt to influence the Unite leadership contest.
Mr McDonnell said: "What he is trying to do is influence the election of the general secretary of Unite and he has dragged the Labour Party into this, completely unnecessarily."
Unite's acting general secretary Gail Cartmail insisted there were "no plans" to affiliate with Momentum and said Mr Watson's attack was "extraordinary".
A statement from Unite clarified that decisions to affiliate with another organisation could not be made by general secretary alone, the union's executive council would also have to agree.
The tape of Mr Lansman's remarks, said to have been secretly recorded at a meeting of a Momentum branch in Richmond, south west London on 1 March, was obtained by The Observer.
Mr Lansman said: "For 20 years the Left was denied a voice. We will deny a voice to no-one. We face big challenges and we need our mass membership to win again."