By Alistair Bunkall, Defence Correspondent
A future coalition government would have to commit to maintaining defence spending if it wants DUP support, the party has told Sky News.
The Northern Ireland party has set the red line as part of its national strategy in the event of a hung parliament.
"We are very clear, whether it be Labour or Conservative, that we need to see a very clear commitment on 2% GDP on spending and that must be upheld. It's not just a NATO requirement, we need this for our defence," the party's defence spokesman Jeffrey Donaldson told Sky News.
It is a significant announcement from a party that could potentially hold the balance of power on 8 May.
The Democratic Unionist Party is seen as a possible ally for the Conservatives in the event of a hung parliament. They are expected to win eight or nine seats at the election meaning their support could swing it for the Conservatives looking to form a government.
They also insist any new government must equip and deploy both of the two new aircraft carriers currently under construction.
The announcement comes on the same day politicians debate the defence budget in the House of Commons. The debate was called by backbenchers after repeated refusals by Downing Street to commit to the budget beyond the next spending review later this year.
One of those taking part in the debate is Conservative MP and member of the Defence Select Committee James Gray.
"The world is an incredibly dangerous place," he said. "We need defence spending, the Americans have made it plain we need defence spending and the Prime Minister is personally committed to doing it, so all we are saying is please live up to what you previously promised."
Tory backbenchers are cross their party is committed to protecting the International Aid budget - a Lib Dem policy - but will not give such commitments to the defence budget.
It is believed the Chancellor is in favour of further cuts to the military, but David Cameron would like to see the budget protected. The MPs hope to put pressure on him ahead of next week's Budget.
Currently Britain is only one of four NATO countries to spend 2% of its national wealth on defence. The others are Greece, Estonia and the United States.
At the NATO Summit in Wales last year, David Cameron forced other member states to pledge to meet the target. Later this year there will be another Defence and Security spending review. If the UK economy continues to grow but the defence budget is not increased, then the UK will slip below that 2%.
The military has already taken a huge hit after the last spending review in 2010.
The Army has shrunk from 102,000 personnel to 82,000 and pessimistic forecasts predict it might be cut further still, taking it beyond a credible force.
Washington is putting pressure on the government not to cut further. Barack Obama has spoken to the Prime Minister in private, but recently the US Army Chief of Staff, General Odierno, publicly warned against further cuts.
