Northern Ireland’s political leaders remained locked in negotiations on Monday night as a deadline for reaching a deal on edged closer.

Talks between the five Executive parties, the British and Irish governments on financial issues as well as matters such as flags and the past were expected to continue late into the night at Stormont Castle.

The participants acknowledge the 11-week process will formally end at the close of this round of discussions, with or without agreement.

Stormont’s DUP First Minister Peter Robinson said the quest to strike a deal was set to go “down to the wire”.

“This is the day decisions have to be taken, whether that rolls into the early hours or not,” he said. “There is no tomorrow as far as this deal is concerned.”

Northern Secretary Theresa Villiers earlier said British prime minister David Cameron's response to the request for a £2 billion financial package to seal a deal was "positive and generous".

Ms Villiers did not disclose the amount of Mr Cameron's latest offer but talks sources said it was in the region of £1.5 billion - a figure that Sinn Féin MP and talks negotiator Conor Murphy did not contradict when he spoke to the press on Monday afternoon at Stormont.

Mr Murphy said the parties were now considering what Mr Cameron had put on the table and would respond later on Monday. But in relation to the overall figure, in terms of actual hard cash and additional loan-raising powers for the Northern Executive, he said “the devil is in the detail”.

Mr Murphy said that last Friday week Mr Cameron said he had offered £1 billion but “when you did the actual accounting of new money it amounted to £10 million”.

“We have to work out what is on offer of additional money and what is your old money repackaged,” he said.

Mr Murphy said the five parties must maintain a united front on the financial package. He did not rule out the parties seeking more funding from Mr Cameron. “The Executive parties are determined to secure the best financial package available,” he said.

“We are all agreed that we will study the paper and come back together to try to reach an agreed position in relation to it, and if needs be go back to the British government on that position,” he added.

The Minister for Foreign Affairs Charlie Flanagan and Ms Villiers began chairing the talks at 12 noon. At that stage Ms Villiers presented the parties with Mr Cameron's offer. "We did some intensive work over the weekend. We believe that our response is a positive and a generous one," she told reporters ahead of that meeting.

Ms Villiers quickly added a caveat, "But it does obviously also have to reflect the constraints we face, the realities of an extremely difficult fiscal situation and, of course, we also need to be fair the rest of the United Kingdom. "

Nonetheless she believed a deal was possible today.

Ms Villiers said Mr Cameron’s offer was conditional on the parties achieving a “broad agreement” on the main contentious matters such as the past, parades and flags” which “always have the ability to poison political relationships and disrupt community relations”.

She did not comment on whether Taoiseach Enda Kenny and Mr Cameron would return to sign off on any deal. Such a visit remained under active consideration, according to sources.

Ms Villiers again insisted that a deal must be struck today although it seemed clear that the talks could at least run on into Tuesday morning. “We can’t go on with this process indefinitely. Right from the start all parties said Christmas is the deadline. If we have not got this sorted by Christmas it is not going to happen,” she said.

“Today has to be it,” added Ms Villiers.

Referring to how a British general election will be held in May she said, “Making these kind of agreements in an election year is extremely difficult.”

The parties financial package involves a request for £200 million to deal with the past; £800 million to cover public sector restructuring, including paying for thousands of redundancies and £800 million to help create a “shared future” in areas such as education and housing.

The parties also want £214 million of penalties for so far failing to implement welfare change to be written off.

Mr Murphy gave a first public confirmation that Sinn Féin would endorse a new welfare system. “The parties have managed to reach agreement in relation to welfare to make sure that the most vulnerable in our society are protected from the welfare cuts,” he said.

He said a "unique system" would be created for Northern Ireland "which the Executive themselves will pay for". This relates to the parties separately agreeing that they can find some £70 million from existing resources to provide support for those who would be hardest hit by cuts in welfare payments.

Mr Robinson earlier said that there was “real money on the table” from Mr Cameron this time. “The devil is in the detail, but at least there is more detail and more money on offer than there was in the previous one,” he said.

He added however that more information was required from Mr Cameron about his offer.

“I consider some progress has been made. It is down to the wire. There is a short period of time left to us to reach conclusions. But we are determined as this is in the benefit of Northern Ireland,” he added.

Mr Robinson also indicated that the Irish Government was willing to provide some financial contribution to any deal. This is understood to refer to proposals to reactivate plans to upgrade the A5 road from the Border at Aughnacloy in Co Tyrone to Derry City.