The EU is willing to extend the transition period after the UK leaves the bloc, Ireland’s foreign minister has confirmed, but warned Britain must not renege on previous agreements to prevent a hard border between the Irish Republic and Northern Ireland.

The confirmation came as the environment secretary, Michael Gove, said a Brexit deal may not be in place until the December summit of EU leaders.

Theresa May will address EU leaders in Brussels for a European council summit before dinner on Wednesday, as negotiators continue to grapple with the drafting of the withdrawal agreement.

A No 10 source said there was no change to the UK’s approach to the transition period, which is scheduled to end in December 2020. “We are not calling for an extension to the implementation period,” the source said.

The UK is pushing for the Irish backstop proposal to be replaced in the withdrawal agreement by a temporary UK-wide customs union, saying it cannot allow Northern Ireland to be “carved off” from the rest of Britain.

The EU27 has said there must be unique “all-weather” insurance protections to avoid any hard border on the island of Ireland, and points to the wording of the December joint agreement that the backstop must operate “unless and until” alternative arrangements to prevent such a border have been put in place.

Before the summit, Ireland’s foreign minister, Simon Coveney, said the EU was willing to offer “new thinking and imagination” to try to close the gap between Brussels and London, but said the UK could not go back on existing commitments.

Quick Guide Brexit and backstops: an explainer Show A backstop is required to ensure there is no hard border in Ireland if a comprehensive free trade deal cannot be signed before the end of 2020. Theresa May has proposed to the EU that the whole of the UK would remain in the customs union after Brexit, but Brussels has said it needs more time to evaluate the proposal. As a result, the EU insists on having its own backstop - the backstop to the backstop - which would mean Northern Ireland would remain in the single market and customs union in the absence of a free trade deal, prompting fierce objections from Conservative hard Brexiters and the DUP, which props up her government. That prompted May to propose a country-wide alternative in which the whole of the UK would remain in parts of the customs union after Brexit. “The EU still requires a ‘backstop to the backstop’ – effectively an insurance policy for the insurance policy. And they want this to be the Northern Ireland-only solution that they had previously proposed,” May told MPs. Raising the stakes, the prime minister said the EU’s insistence amounted to a threat to the constitution of the UK: “We have been clear that we cannot agree to anything that threatens the integrity of our United Kingdom,” she added.

“The EU side is willing to allow more time in the transition period to agree an alternative solution to the backstop,” he told BBC Radio 4’s Today programme. “What [the EU chief negotiator] Michel Barnier is now suggesting is: let’s ensure the backstop is never likely to be used by creating the space and time for the UK and the EU to be able to negotiate UK-wide customs arrangements.”

However, Coveney said, the backstop would not work as an insurance option if it were time-limited. “This is like if you take out fire insurance on your home … It’s there to reassure people there is a fallback position if all else fails,” he said.

Coveney said EU countries including his had no desire to see the breakup of the UK, or to impose a customs border down the Irish Sea, but added that British opponents also had to respect what had already been agreed.

“Nobody is asking that Northern Ireland remain part of the EU,” he said. “But what we are doing is recognising that Northern Ireland is different to other parts of the United Kingdom. A peace agreement for the last 20 years has provided stability and normality on the island of Ireland, and an absence of violence – that is what we are trying to protect.”

Extending the Brexit transition would leave the UK paying billions more to the EU, while having no say over how the money is spent.

Under the Brexit transition terms, the UK will remain subject to EU rules and the writ of the European court of justice, without any say or representation in EU institutions. Extending the transition would mean an extra year of contributions to the EU budget, without any British leader at the table when the EU agrees the rules for the 2021-27 budget.

Barnier has suggested the UK government had failed to meet the conditions necessary for a special November Brexit summit to be called, though Coveney suggested on Wednesday that it was still a possibility.

Gove, one of the most high-profile Brexiters in the cabinet, told a committee of MPs it was possible a deal would not be reached until December.

“It might be the case – we’ve seen how European negotiations work – that progress is made at October council, progress is made at the November council, and it is even as late as the December council before a deal is done. Obviously, everyone wants the deal to be done earlier,” he said.

Failure to reach a deal until December would put the government substantially behind schedule. Ministers had hoped to put a final agreement before parliament for a vote before the Christmas recess.

Coveney said the EU would begin publishing its contingency planning for a no-deal scenario next week, describing it as “not a pretty picture”.

No 10 said May would meet Ireland’s prime minister, Leo Varadkar, as well as the European council president, Donald Tusk, and the European commission president, Jean-Claude Juncker, for talks, then address the EU27 leaders before dinner. She will not attend the rest of the meal, and will depart for the UK ambassador’s residence in the city as Brexit talks continue among EU leaders in her absence.

May has not ruled out the possibility of the UK paying into EU funds in the event that a trade deal cannot be agreed for after the transition period.

The Labour leader, Jeremy Corbyn, asked May in the House of Commons to “confirm the Treasury legal advice given to cabinet that in the event of no deal that the government would still have to pay a divorce bill of £30bn”.

She said the UK had agreed in principle to pay the exit bill of £39bn in December subject to a successful conclusion of negotiations.

May received a letter from Donald Trump’s trade representative on Wednesday, telling Congress the US wants to start formal trade talks with the UK “as soon as it is ready” after Brexit.