The EU summit in Brussels had been due to come to a close this afternoon but discussions looked set to continue late into the evening and leaders were advised to book hotel rooms for an extra night.
A proposed “English breakfast” on this morning for all 28 countries to thrash out the final elements of the deal as was repeatedly postponed as Mr Cameron engaged in a series of meetings with individual EU leaders.
In a sign of the frustration with the UK on the part of some EU leaders, Czech Europe minister Tomas Prouza wrote on Twitter: “As the time passes, I am more and more perplexed by the British approach of non-negotiation. Quite unorthodox, to say the least.”
But Polish Europe minister Konrad Szymanski sounded a more optimistic note, insisting a deal was “close”.
He said: “We managed to have a compromise on many, many issues ... but we still need more clarification, more guarantees to get the compromise which would be satisfactory for both sides. There’s still some way to go, we need some hours.”
Mr Cameron had earlier told reporters he was “happy to stay until Sunday” if need be, adding he had warned wife Samantha and their children he may not be home this weekend.
While European Council president Donald Tusk published a draft agreement on the changes demanded by the UK, fine detail remained to be agreed.
Eastern European countries have expressed particular concern over proposals to limit in-work benefits to EU migrants.
Britain was also under pressure to accept a clause in the agreement preventing future attempts to make fresh demands.
As the Brussels talked continued, Yorkshire MEP Linda McAvan today launched the Britain Stronger in Europe group’s campaign in the region at a Sheffield business.
The ‘in’ campaign argues leaving the EU would make it harder for Yorkshire firms to trade in Europe.
Ms McAvan, one of Yorkshire’s six MEPs, said: “When people actually think about the decision that they are going to have to take, which is a very serious decision, they are going to think about their own jobs, the people they know, their families our economy and our region which is a big manufacturing economy.
“We do export and I think we will hear more business voices saying ‘hang on a minute’ this is a very serious issue.”
An agreement between the Prime Minister and fellow EU leaders on changes to Britain’s membership terms is expected to trigger a referendum in June.
Ms McAvan said: “There is a danger of people bandying around this number, that number, that will make people make people fed up of listening about it.
“I think we have to keep coming back to the enormity of the decision. It is an enormous decision, there won’t be a second referendum. There’s a lot of talk of David Cameron’s reforms and the nitty-gritty but we’re not voting on his reforms, we’re voting whether or not to stay in the European Union.”
The launch was held at Diamond Dispersions, a Sheffield company employing 25 people, which produces highly-specialised ink pigments and trades with firms across Europe.
Director Sue Wright said: “We export all over the world, we are very proud of our European heritage here. We’re proud to be British and proud to be part of Europe.
“It helps us when we are talking to people in China, when we are talking to people in South Korea. It helps is in Germany because we can just sell to them with an invoice and a safety data sheet, the same as selling to someone in Manchester.
“It is very important that we can carry on trading the way we trade with as few tariff barriers against us as possible.”