Any withdrawal agreement is likely to require a backstop, Labour’s shadow Brexit secretary, Keir Starmer, has said, admitting for the first time that a renegotiation was unlikely to be possible in the time before the UK is set to leave the EU.

Starmer has previously been highly critical of the backstop, which would keep Britain in an effective customs union with the EU as an insurance policy to prevent a hard border in Northern Ireland, with no unilateral right for the UK to exit until a technological solution to keep open the border can be found.

However, speaking on the BBC’s Andrew Marr Show on Sunday, Starmer suggested Labour was open to keeping the terms in the withdrawal agreement as long as other key changes were made – such as a commitment to a full, permanent customs union and a high alignment with the EU’s single market.

“At this stage any deal probably does require a backstop and we’ve got to recognise that,” he said. “There are problems with this backstop and we have got to recognise that but because we are in this stage of the exercise, nearly two years in, the chances now of a deal that doesn’t have a backstop are very, very slim.”

The EU has repeatedly said changes cannot be made to the legally binding withdrawal agreement struck by Theresa May and EU leaders and which contains the terms of the backstop.

It is more likely that changes could be made to the non-legally binding political declaration, which sets out the direction for the future relationship between Britain and the EU.

A Labour source said it had long been the policy that an insurance option would be needed for Northern Ireland but viewed it as far less likely to be used if the intention was to negotiate a permanent customs union arrangement.

The source said Starmer’s view was reflecting the reality of the short time period available and that the majority of changes Labour would be seeking would be to the political declaration, as well as some clarification on the scope of the withdrawal agreement.

Starmer said the prime minister must be prepared to shift her red lines in order for constructive discussions with Labour – but if that was not possible then the party should consider backing a second referendum.

“If she won’t do that, it’s very hard to see where we go from here. That’s why at conference we said, to break the impasse, if we get to an impasse, that’s why you need an option of a public vote,” he said.

Starmer denied Labour would automatically seek to block any deal the prime minister proposed. “I have said for two years we will faithfully look at any deal that is brought back which is what we did on Tuesday,” he said.

“If she stood up on Tuesday [after the meaningful vote] and said she was genuinely now open to looking at other options – but she hasn’t done that. She is the block.”

The Labour leader, Jeremy Corbyn, has said his party will not open discussions with the prime minister until she pledged that she would not take the UK out of the EU with no deal.

“If she moved into that space and said, my red lines have gone, I’m not going to hold a gun to your heads about no deal, that would shift the position incredibly,” Starmer said.

The shadow Brexit secretary stressed again that he believed an extension to the article 50 timeline was inevitable.

“It’s extremely difficult to see how the prime minister can achieve what needs to be achieved in 68 days and therefore I think it is inevitable article 50 is going to be extended. And the blame with that lies with the prime minister,” he said.