ED Miliband will today praise union leaders backing No - days after the RMT transport union became the first industrial union to come out in favour of a Yes vote.
The Labour leader will make his comments at an annual dinner organised by the Trades Union Congress, which has itself stayed neutral in the debate.
Yesterday the leader of the TUC said she thought that workers were "better together" as she predicted Scots would consider the fate of working people across the UK as they cast their vote on September 18.
Frances O'Grady had earlier warned Conservative plans after 2015 would effectively ban strikes "by the back door".
She said: "Our values are always that working people are better together."
As the TUC congress opened in Liverpool, she added that independence "clearly has big implications for working people everywhere in the UK - and no doubt Scottish people will be thinking about that when they cast their vote".
But she insisted: "This has to be a matter for the Scottish people to decide."
Mr Miliband will single out those unions that are fighting alongside Labour for a No vote in his speech to the TUC general council dinner in Liverpool.
He will also forecast that those unions, which include the GMB, will play a key part in winning the referendum.
He will say: "The Labour movement was founded on the principle of solidarity. You know that unity is strength.
"You know that we achieve more together than we can do alone."
A number of unions have remained strictly neutral, including the UK's largest union, Unite.
Mr Miliband will also appeal to union leaders for help to win next year's General Election,
He pledged a Labour government after 2015 would argue for causes close to union members' hearts, saying it would be "a Labour government that knows the battle against low wages, for equal pay, and to end the abuse of zero-hours contracts is one for the UK as a whole.
"A Labour government that cares about fairness and the future of our public services whether you live in Doncaster or Dundee."
And he will reiterate Labour's pledge to freeze energy bills, raise the minimum wage and introduce fairer taxes.
Ms O'Grady highlighted Conservative calls for a turn-out threshold in strike ballots, and new specific criminal offences for people on picket lines.
She said: "These proposals are designed to make unions weaker. And if unions become weaker, then the chances of people winning a pay rise, improving living standards and tackling inequality in Britain today will become a good deal harder.
"The Conservative Party is not just proposing a few more bureaucratic obstacles that will make life a bit more difficult for trade unions.
"Taken together, they would effectively ban strikes by the back door. And, on top of that, they would open up elected union leaders to increased surveillance by the state."
Bob Thomson of Labour for Independence said: "We already know that some 230,000 Labour voters - around one in three - are already planning to vote Yes, and we are aiming for a majority a week on Thursday. Members of the RMT union have recently voted to support a Yes vote, and the Scottish Prison Officers Association members also back Yes."
Mr Thomson, a former chairman of the Scottish Labour Party, also referred to the Scottish Trades Union Congress, which has a neutral stance on independence.
He said: "The STUC's stated position - which is highly significant - is not to support the anti-independence campaign, and the STUC have also said trade unionists are 'more attracted' to Yes.
"It is our job and intention between now and September 18 to persuade a majority of trade unionists of the opportunities of Yes to build a fairer and more socially just Scotland, to protect and enhance workers' rights, and to create new jobs by growing our economy in a sustainable way."
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