The Remain campaign is facing the "very real prospect" of defeat in the referendum in two weeks' time, a senior Labour figure has warned.
Shadow home secretary Andy Burnham said a vote to leave the European Union on June 23 could lead to social "fragmentation" and the break-up of the United Kingdom.
He also sharply criticised the party's campaigning saying it had failed to reach out to traditional Labour voters amid fears that concerns about immigration are driving them to back Leave.
"We have definitely been far too much Hampstead and not enough Hull in recent times and we need to change that. Here we are two weeks away from the very real prospect that Britain will vote for isolation," he told BBC2's Newsnight.
"I think it would have a profound effect on our national life - the fragmentation that will come, the fear and the division.
"Those are all the things that the terrorists couldn't create with their bombs and yet we will have a situation where society becomes more divided.
"If this decision is taken, dominoes will start to fall. It won't just be the EU that starts to break up, it will be Britain too."
His intervention came as the prominent Labour backbencher John Mann announced that he was joining the handful of Labour MPs who are supporting Leave.
Writing in The Sun, he said that he now favoured withdrawal as the country had "one hand tied behind our back" when it came to controlling immigration in the EU.
"People have been terrified about talking about immigration. But on polling day they are going to get a big shock across the country," he wrote.
"They are going to get a big shock about how Labour councillors vote, they will get a big shock about how Labour members vote.
"And it shouldn't come as a shock how many Labour voters will vote. Because a people's revolution is under way. This is about returning power to the people."
The warnings come as Ed Miliband tries to inject new momentum into Labour's campaign effort with an attack on Boris Johnson and the Leave camp for perpetrating a "fraud" on the British people.
The former opposition leader and other senior colleagues will make a series of interventions aimed at winning over wavering Labour supporters.
Former cabinet minister Yvette Cooper will release a report warning of the damage facing Labour heartlands if the "far right of the Conservative Party" gets its way.
And deputy leader Tom Watson will release analysis indicating Brexit could result in £18 billion of welfare cuts and tax hikes as the Tories impose tighter austerity measures.
The attacks by Labour are aimed at rallying the party's supporters behind the Remain cause following criticism of leader Jeremy Corbyn's efforts in the referendum campaign.
In a speech in London, Mr Miliband will warn that senior members of the Brexit campaign want to abolish measures protecting workers' rights.
He will say: "The Leave campaign are trying to perpetrate what I can only describe as a fraud on the British people. Tories who in the last days of this contest are trying to disguise themselves in Labour clothes.
"Let's be clear what the Leave agenda would mean for working people. They want out of Europe so we can be out of the social chapter, as Boris Johnson said in terms in 2012. Their competitiveness strategy for Britain is deregulation and the erosion of rights of working people."
A Vote Leave spokesman said: "As support drains away from the Remain campaign, they are getting ever more desperate and hysterical with their fanciful Leave predictions.
"We need to vote Leave if we want to take back control of our economy, borders and democracy."