Significant progress is still needed in the Brexit negotiations to flesh out the final political declaration on the future relationship, Downing Street has said, including addressing some language on customs that alarmed Brexiters.

Theresa May would play a negotiating role in the final stages before Sunday’s summit in Brussels, her spokesman said, although there is no day set this week for her anticipated meeting with the European commission president, Jean-Claude Juncker.

The prime minister said over the weekend that she intended to go to Brussels this week ahead of the emergency summit on Sunday. No 10 denied that the threat of a possible no-confidence vote was the reason no date had been finalised.

“It is dependent on the negotiations,” her spokesman said. “This is clearly a very intense part of the negotiations. You would expect the prime minister to be working very hard to get the best deal for the UK and her engagement with the commission president is part of that.”

A Downing Street source said they anticipated the final political declaration, which sets out aims for the future relationship with the EU alongside the withdrawal agreement, still needed substantial work. It is currently seven pages long in draft form, but officials said they expected the final version to be significantly longer.

One phrase in the political declaration which alarmed Brexiters says both sides have committed to “combining deep regulatory and customs cooperations, building on the single customs territory provided for in the withdrawal agreement”.

Critics had suggested it was a step towards a permanent customs union. That is one of several areas, as well as security, where a No 10 source said there was still work to be done.

“We are adding more detail to the outline political declaration for it to become the future framework,” May’s spokesman said. “There is a lot of work to be done. The PM has described this period as ‘critical’ and I would expect the talks this week to be intensive.”

Negotiations this week should also provide clarity on the possible length of the transition period, should no agreement be in place by the current end date of December 2020.

The text of the agreement states that the period may be extended to “20XX” and it remains unclear whether this will be amended to show a fixed maximum date.

Earlier on Monday, the business secretary, Greg Clark, suggested that could be until December 2022. That idea had been raised by the EU’s chief Brexit negotiator, Michel Barnier at a meeting of ambassadors from the EU’s 27 member states.

However, the prime minister’s spokesman said she had been clear that the UK needed to have the agreement finalised before the next general election – the latest date that can be held is June 2022.

“The prime minister has always been clear that it is important the implementation period ends in advance of the next general election and that remains the case,” he said.

No 10 is continuing discussions with Tory backbenchers concerned about the deal. Among other things, Downing Street will assure them that there are legal ties between the withdrawal agreement and the future framework which could ultimately lead to the UK suspending its final payments to the EU should Brussels attempt to back out of any of the agreement on the future relationship.

“If either side fails to honour the commitments it has made in the political declaration then there are stages you can go through, ultimately leading to the suspension in part of parts of the withdrawal agreement, including financial payments,” May’s spokesman said.