The leader of Britain's biggest union warns that it could further slash financial support to the Labour party – and face internal pressure to disaffiliate – if Ed Miliband's cash-strapped party forms a coalition with the Liberal Democrats after the next election.

In an interview with the Observer on the eve of the TUC Congress in Liverpool, Len McCluskey, general secretary of Unite, says he will demand that Labour govern alone if it wins the election but falls short of an overall majority.

He argues that Unite's 1.4 million members will not look kindly on a Labour party that refuses to run a minority government and teams up instead with Nick Clegg's Liberal Democrats. He fears a coalition would deliver more austerity and dilute Miliband's ability to deliver a "bold and brave" agenda for working people.

Asked if Unite, Labour's biggest financial backer, would continue to pour cash into Labour coffers if there was a Lab-Lib government, McCluskey said: "If Labour win – and I hope they win, in fact I believe they will win – if they enter into a coalition there is no doubt it would lead to a discussion about our relationship with Labour. I think it would raise serious issues within the union. In Unite within our rule book we are affiliated to the Labour party, so any argument about disaffiliating from the party would have to go to a rules conference – that is, a huge conference of our activists who would have to debate that.

"To enter into a coalition in those circumstances would be very, very dangerous, given the way the Liberal Democrats have played during the coalition."

The former Liverpool docker says another coalition would spread more disaffection with the political process among voters because they would conclude "whichever way you vote you end up with the same, Nick Clegg or whatever".

The only circumstance in which he could see Labour having to consider working with the Lib Dems, as a last resort, would be in the aftermath of a general election which followed a yes vote in the Scottish independence referendum this month.

In a scenario in which Labour went on to win the most seats in 2015, but then lost its Scottish MPs when independence was declared in March 2016, he suggests some accommodation with Clegg's party might have to take place.

Unite, which has donated £12m to Labour since the 2010 election, is ready to give more financial backing to Labour ahead of the general election. But it has already moved to change the way it funds the party following Miliband's reforms of the union funding link. Earlier this year it cut the amount it hands over in automatic affiliation fees by half to £1.5m a year, while pledging to try to build up the number who actively opt to back the party. The GMB union decided last year to cut its affiliation fees by 90%.

Miliband's changes, to be introduced over five years, were announced after a row over the selection of a parliamentary candidate in Falkirk involving Unite. They initially received a hostile reception from some trade union leaders, who feared they would dilute their power, although McCluskey came out in support. His comments on funding a Lab-Lib administration will alarm Miliband, and underline McCluskey's determination to remain a key player in party decision-making up to and beyond the May 2015 election.

The TUC will hear demands from unions, including Unite, for a return to more collective bargaining as the way to improve the pay of working people. "Wealth is still being created, but it is not being distributed," McCluskey says. "What can we do to arrest the decline? One way is to restore collective bargaining. In Germany 90% of workers are covered by collective bargaining."

Congress will also highlight what McCluskey says is the "frightening" consequence of the Transatlantic Trade and Investment Partnership, an agreement between the EU and the US that is currently under secret negotiation, which he says will allow parts of the NHS to be sold off to private companies abroad, without any possibility of a future government restoring them to public ownership.

He is worried Labour will endorse an "austerity-lite" programme. "While we understand they [Labour] have to be careful, the basis of our argument is that austerity is not working … The best stimulus you can give is giving low-paid workers a pay rise. They would not put any of it in the Cayman Islands." But while challenging shadow chancellor Ed Balls, McCluskey does not question his suitability as chancellor-in-waiting. He wants Labour to win and does not intend to rock the boat more than he needs to keep his members happy. "It is not up to me to choose Ed's team," he says.

McCluskey seems genuinely angered by his portrayal in the rightwing press as a dangerous extremist and by the depiction of unions such as Unite as wholly negative forces in public life. He says he is immensely proud of what Unite has achieved in constructive talks with companies such as Jaguar Land Rover, General Motors and Rolls-Royce, which have helped to keep factories open and secure jobs.

He wants to do more to portray unions as positive forces, and to show that on many occasions they work well with company bosses, for everyone's benefit. "95% of our time we are engaged positively," he says. "We are a legitimate part of our society, we are by far and away the largest voluntary organisation, we speak on behalf of a huge section of the population and their families."