Nigel Dodds sets out DUP's demands
The Westminster leader of Northern Ireland's Democratic Unionist Party has said an Ed Miliband government would have to offer a European referendum in exchange for the support of DUP MPs in the event of a hung parliament.
He also indicated the DUP rules out any coalition involving the SNP.
As election forecasts continue to indicate the next election may produce another hung parliament, this time requiring not just two parties to provide a Commons majority but possibly three, Nigel Dodds set out the party's thinking on the prospect.
He told BBC Newsnight the party "potentially has a very strong hand".
The DUP is currently the fourth largest in the Commons with eight seats.
Mr Dodds said he believed it would be "very, very difficult" for his party to support any coalition involving the SNP.
Sources later said there would be no circumstances in which the DUP would sit alongside the SNP.
He said Ed Miliband would have to offer a referendum on Europe to win DUP support for any government he may try to form after the election.
He also set out that the price of a coalition with the Tories would be reversing the so-called bedroom tax across the UK.
Mr Dodds said: "We're very clear, of all the changes to the welfare system the bedroom tax is the most iniquitous one.
"That's why we have dealt with it back home in Northern Ireland.
"We believe it would be the clearest, strongest signal to the country as a whole that the deficit is being addressed but it is being done in a compassionate and fair way."
With the results of May's general election looking increasingly finely balanced, which party the DUP might support has become a matter of growing interest.
According to forecasts of May's results run by people at Election Forecast on behalf of Newsnight - the "Newsnight Index" - in a third of simulations, no two parties have a majority, meaning the role of smaller parties is coming to the fore.
In the Guardian on Thursday morning, Mr Dodds set out a three-point programme for government that a party must meet to get the support of the DUP - focusing on hitting defence spending of 2% of GDP, reversing the "bedroom tax" across the United Kingdom and action to better protect the UK's borders.
He told Newsnight: "We don't presume to put ourselves in position of parties with more seats than us, but we do recognise that we could be in a pivotal position.
"At the end of the day, we're not giving a carte blanche, we're not giving anybody a signal that they can do whatever they like and presume on our support.
"We'll judge the issues based on what is best for the union and for Northern Ireland."
Asked whether he would rule out accepting a role in government, he said: "It's not something we seek, it's not on our agenda.
"I think potentially we have a very strong hand. And I think people of the UK deserve to know from parties like us not just where do we stand on Northern Ireland issues, but they deserve to know what is the DUP's vision for the UK as a whole - strengthening the union, withstanding the tide of nationalism, deficit in a fair and compassionate way and relationship with the EU."
Pressed on whether he would withhold support for Mr Miliband unless he delivers a European referendum, Mr Dodds said: "I think an EU referendum needs to be part of what is on offer to the British people after the election, whoever is prime minister."
When questioned if he ruled out supporting a government that included the SNP, Mr Dodds said: "Yeah, yeah I think it would be very difficult to imagine circumstances where the DUP would be prepared to support a government of which the SNP was part."
Mr Dodds said the SNP was "fundamentally out to break the United Kingdom" and that it would be a "tragedy and a grave mistake" for Labour to enter into an arrangement with the party.