Downing Street has insisted the government will not sign up to any deal with the EU that would keep the UK in a customs union indefinitely, as the prime minister tries to assuage concerns in her cabinet.

British negotiators in Brussels are thought to be preparing to sign up to a draft of the backstop for Northern Ireland – aimed at preventing a hard border – that could see the whole of the UK effectively inside the customs union.

The EU27 have long signalled they will not accept any end date for the backstop being inserted into the legally binding text of the withdrawal agreement.

But Theresa May hopes to convince sceptical ministers, several of whom expressed concern about the idea at an informal meeting in No 10 on Thursday, that the backstop will never need to be invoked.

The prime minister’s spokeswoman told journalists on Friday morning: “The prime minister would never agree to a deal which would trap the UK in a backstop permanently.”

However, she declined to say whether any end date could be included in the agreement, the final details of which are being decided in Brussels over the weekend.

“When we published our plans in June on a UK-wide customs backstop, we were absolutely clear that the arrangement would be temporary, and only in place until our future economic relationship is ready,” the spokeswoman said.

“Our position is that this future economic relationship needs to be in place by the end of December 2021 at the latest.” This date is the end of the transition period negotiated by the UK as part of the divorce deal with the EU.

However, her remarks appeared to contradict comments from the chancellor, Philip Hammond, who is in Bali for the International Monetary Fund and G20 summits.

Asked whether the UK could stay indefinitely in a customs union, Hammond said: “We’re not going to remain in anything indefinitely, we’re very clear this has to be temporary.

“[But] there needs to be a period, probably following the transition period that we have negotiated, before we enter into our long-term partnership, just because of the time it will take to implement the changes required.”

Speaking in the House of Commons earlier this week, the Brexit secretary, Dominic Raab, said any backstop must be “temporary, limited and finite”.

Brexit-supporting ministers are focusing on which mechanism will be used to ascertain whether the backstop needs to be invoked, and when it will come to an end.

They fear the arrangement could end up becoming Britain’s permanent economic relationship with the EU – despite the fact that government policy is to leave the customs union – as it would limit the UK’s ability to strike future trade deals with non-EU economies.

May hopes British negotiators can reach an agreement over the weekend, which she can discuss with the cabinet on Tuesday. Her spokeswoman said the meeting on Thursday was one of a number held with groups of key ministers.

Downing Street also announced a further 29 no-deal notices would be published on Friday afternoon, setting out what businesses and consumers should do in the event of there being no agreement by 29 March 2019.