Labour’s deputy leader said he is “fighting for the future existence” of the party as the row over a hard-left plot intensified.
He told the BBC: “I regard this as a battle for the future existence of the Labour Party.
This is high stakes and I hope my fellow members are going to understand that, and our leader.”
Mr Watson spoke out after veteran London left-winger Jon Lansman, who founded Momentum, was recorded telling supporters he expected Unite to affiliate with Momentum if union boss Len McCluskey is re-elected as General Secretary next month.
The move would formalise the campaign group - set up in 2015 to support Mr Corbyn - as a portion of the fees paid by Unite’s one million members would go towards helping its work.
Mr Watson said he wants Mr Corbyn to “deal with” Mr Lansman and stop Unite from channelling members money into the organisation that “wants to take control of the Labour Party”.
He also took aim at Mr Lansman’s focus on party control rather than helping Labour win the next General Election.
He said: “I’m afraid there are some people who don’t have our electoral interests at their heart and they’ve got to deal with it and that’s why I am speaking out now.”
Christine Shawcroft, director of Momentum, said they are “moderates” and the political labels used by Mr Watson were “non-sensical”.
She then described him as someone who wants to go back to the Blairite “command and control” party structure that no longer fits with the party’s membership.
Speaking to the BBC, she said: “There’s a democratic deficit in the Labour Party, where the structures of the party don’t reflect the support for Jeremy Corbyn that has been shown by the mass of the membership.”
“This is about democratising the Labour Party, which will strengthen the Labour Party.”
Ms Shawcroft added that she hoped Unite would decide to affiliate to Momentum, saying: “The offer is always open for any organisations, trade unions, that wish to affiliate to Momentum.
“It is not a question of anyone disaffiliating from the Labour Party and affiliating to Momentum instead.”
Momentum founder Mr Lansman was secretly recorded addressing members in Richmond earlier this month and discussed ways to ensure the left is represented in all key positions of power if there is another leadership contest.
He is also said to have told activists it was “absolutely crucial” that they secured a change to the party’s rules to ensure that whenever Mr Corbyn stands down, they are able to get a candidate on to the ballot paper to succeed him.
Currently, a candidate must obtain the support of 15 per cent of Labour MPs and MEPs in order to stand - a threshold a new left-wing contender is unlikely to be able to achieve.
This change will be debated at the party’s Autumn conference, with those on the left and right of the party planning on sending as many delegates to take part in the vote as they can.
Unite’s acting General Secretary, Gail Cartmail said: “Tom Watson has made claims about Unite and its General Secretary Len McCluskey which are entirely inaccurate.
“As Unite has made it clear it is exclusively for our executive council to determine which organisations we affiliate to. There are no plans for Unite to affiliate to Momentum. For the record, Len McCluskey has never met Jon Lansman to discuss this or any other matter.”