Donald Tusk, the president of the European council, has said the “foundations” of a Brexit deal are now ready for approval by EU leaders but that Brussels is waiting on London after late “doubts” in Downing Street.

The deal due to be signed off by leaders at a summit on Thursday would involve the drawing of a regulatory and customs border down the Irish Sea. The Democratic Unionist party has made public its opposition to the terms of the tentative agreement with its leader, Arlene Foster, insisting that more discussions are needed.

Tusk said it would be clear by the end of Wednesday whether Johnson had the political backing to go ahead with the revised withdrawal agreement and political declaration on the future relationship with the bloc.

He told the Polish broadcaster TVN24: “The basic foundations of an agreement are ready and theoretically tomorrow we could accept this deal with Great Britain.

“Theoretically in seven to eight hours everything should be clear. Yesterday evening I was ready to bet that it’s all set and agreed, today there are certain doubts on the British side.”

Earlier in the day the EU’s chief negotiator, Michel Barnier, had spoken of his optimism that a Brexit deal could be reached by the end of Wednesday.

Despite the last-minute threat posed by the DUP’s opposition to Downing Street’s plans, the EU’s chief negotiator told Jean-Claude Juncker’s team of commissioners on Wednesday morning that he believed a deal could be salvaged in the next few hours.

A key meeting of EU ambassadors with Barnier that was set for 1pm London time was pushed back to 4pm to allow extra time for Johnson to win over the unionist party. It was then delayed to 6pm, as talks in London continued with the DUP.

“We are still waiting for a signal from across the Channel,” said one EU diplomat. “A lot of things are happening between the cabinet, DUP, and the hardest Brexiters.”

Despite the prospect of a deal being struck in Brussels, EU sources said the prime minister’s pledge to leave the EU on 31 October “do or die” remained in doubt even if an agreement was secured.

EU diplomats said the last-minute nature of the talks meant that time would be required after the summit for governments and parliaments to scrutinise the legal text of the deal.

As a result, the EU’s leaders will only give their political rather than full formal agreement to the deal when they meet on Thursday and Friday.

The EU27 will then want the Commons on its “super Saturday” session to give its assent to the revised withdrawal agreement and political declaration.

Formal EU agreement would be given at a later date once the capitals had time to work through the legal text. Sources in Berlin have suggested that process could take as long as two months.

“It is clear that there can only be a political agreement tomorrow or Friday because we cannot see any text. We cannot say ‘yes’ without legal scrubbing,” a diplomat said. “We need much more time.”

The diplomat added that the leaders would require “clarity from the House of Commons on Saturday because we’ve had three deals before”.

“This is the fourth time they want clarity from the UK side. We won’t continue with legal scrubbing and translations to end up with something that is going to be voted down. There is a clear desire for an outcome in London that shows support,” the diplomat added.

A second diplomat said the EU “wanted guarantees”. “Will there be a vote that will show there is a majority for a deal?” the source added.

Under the deal being negotiated, Northern Ireland would not be part of the EU’s customs territory, but the bloc’s full customs code would have to be enforced in the Irish Sea.

“Northern Ireland would de jure be in the UK’s customs territory but de facto in the European Union’s,” one diplomatic source said of the tentative agreement.

Beyond the DUP’s issues with the deal, it is understood the talks between British and EU negotiators have run into trouble on the issue of level playing field guarantees.

The guarantees are promises to maintain EU standards in environment, tax and state aid to prevent Britain from undercutting the EU for a competitive advantage.

The guarantees are part of the political declaration agreed by Theresa May, which sets out the broad terms for future free trade agreement talks, but Johnson’s chief adviser has sought to have the reference removed from the text.

A diplomat said EU leaders would not agree to the revised withdrawal agreement separately to the political declaration.