The SNP would be prepared to accept Gordon Brown's idea of a US-style elected senate of the "nations and regions" to replace the 700-year-old House of Lords as part of a deal to prop up a Labour minority government, senior party sources have indicated.

The move is another sign that the SNP could help Ed Miliband into Downing Street in the event of a hung parliament in May.

It is also one of the first indications of where the parties could find agreement during any post-election negotiations. Earlier this week, First Minister Nicola Sturgeon paved the way for an easing of relations with Labour on healthcare, saying SNP MPs would vote on England's NHS to stop creeping privatisation that could harm Scotland's NHS.

The Nationalists have already set out a number of key concessions they say Labour would have to make for any deal to go ahead, including the scrapping of Trident.

But a deal on a senate would come with strings attached.

The SNP, whose MP Pete Wishart last week led a Commons debate on abolishing the Lords, would demand swift action, possibly by as early as 2017, the sources indicated.

The leadership is currently looking at areas where both parties' general election manifestos are likely to agree.

A senior party source said that if the SNP held the balance of power, that "would show that things have changed, in politics and across the UK, and we should all respond to that changed situation".

He added: "We are completely committed to getting rid of the unelected House of Lords and could be open to the idea of a senate of the 'nations and regions'."

The move could lead, for the first time, to SNP members in Westminster's second chamber.

The party has always refused to sit in the unelected Lords, claiming it is an undemocratic anachronism. In the past, it is thought some of its senior figures preferred a unicameral arrangement along the lines of Holyrood. During the referendum campaign, it argued that independence would remove the problem for Scotland, but it now is having to turn its mind, in wake of the No vote, to what should happen on reform.

SNP sources have also indicated that they would expect any coalition negotiations to last "considerably longer" than the five days it took the Conservatives and Liberal Democrats to agree a deal in 2010.

Similar talks in other European countries can last for weeks if not months.

In a parallel move, the prospect of elected Westminster senators chairing Holyrood committees is being considered by a cross-party group of peers as part of a move to reform radically Britain's constitution.

Earlier this week, members of the cross-party House of Lords group met William Hague, the Commons Leader, who is expected to publish the Conservatives' preferred option on English Votes for English Laws (Evel) shortly.

Liberal Democrat peer, Lord Purvis of Tweed, said the Tory frontbencher made clear that he was "open-minded" about the creation of a UK constitutional convention but stressed its establishment should not be a means of delaying Evel.

Lord Purvis said he was hopeful that a form of words could be agreed by David Cameron, Nick Clegg and Ed Miliband and placed in each of their parties' respective manifestos to deliver a convention following the General Election.

The draft wording, seen by The Herald, would commit all three parties to setting up a an independent body, which would report on the "relationship between the nations and all parts of the UK" and "arrangements for the governance of England".

Half of those who sit on the convention would be members of the public, it adds.

Lord Purvis said the outline plan was that legislation would be passed by the summer of 2017 to create "a Charter of the New Union".

However, he suggested that the new senators would not be in place until after the next General Election.

"By 2020, we would have an elected second chamber, that was reformed, renewed and modernised. The new-look Senate, replacing the House of Lords, would be brought in after the 2020 election under a Charter for a New Union that would stand for the long term."

A number of options of an elected second chamber are on the table. One is to use the German system, whereby the respective parliaments themselves elect senators representing the four constituent parts of the UK.

Lord Purvis revealed one suggestion being considered was that Scottish senators could be chosen to chair Holyrood committees, thereby ensuring a "check and balance" on the executive and creating an inter-parliamentary link, that would encourage the cross-fertilisation of ideas between Westminster and Holyrood.