Labour has insisted that a Brexit withdrawal deal must include an Irish border backstop.
Shadow Brexit secretary Sir Keir Starmer said that with the clock ticking down to the UK's departure from the EU, it was "impossible to see a way forward without a backstop".
During a visit to Belfast, Sir Keir said while Labour had difficulties with the form of the backstop in the current deal, he and colleagues acknowledged there was a need for such an insurance policy in principle, to ensure a free-flowing Irish border post-Brexit.
"From the Labour Party point of view, we obviously see that there are difficulties with the backstop," he said.
"There are features there that are going to cause concern. But we recognise there is a need for a backstop at this stage of the exercise."
His comments come as Labour has faced calls to clarify its position on the contentious issue.
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Last week, Labour MPs were whipped to vote against a Westminster amendment calling for "alternative arrangements" to replace the backstop.
However, last week party leader Jeremy Corbyn also said he had a "problem" with the backstop proposal, insisting it was unacceptable that the UK could not unilaterally decide to withdraw from it.
The purpose of the backstop arrangement is to ensure the Irish border remains open, even if a wider trade deal between the UK and EU fails to materialise at the end of the Brexit transition period.
It would effectively see the whole UK continue to adhere to the customs union, while Northern Ireland would also align with EU single market regulations on goods, meaning it would operate under a differing legal framework to Great Britain.
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On Monday, Sir Keir met business and trade union leaders in Belfast, as well as leadership figures from Sinn Fein and the SDLP.
He told the Press Association: "The prime minister has effectively run down the clock and therefore it is impossible to see a way forward without a backstop.
"So, whilst we have got concerns that we have set out about the backstop, we do accept the principle that there has got to be a backstop."
Sir Keir said he did not believe Theresa May would succeed in agreeing an alternative to the backstop with the EU, stating: "The prime minister and her team have spent over 12 months trying to find an alternative to the backstop.
"We have only got the backstop because they couldn't find an alternative.
"So for her to go back now saying, 'I don't want the backstop, I want an alternative' is to stand the last 12 months on its head.
"And I think that's what is causing the anxiety because nobody realistically thinks she's going to succeed in that objective."
After the meeting with the shadow Brexit secretary at the Europa Hotel, Sinn Féin vice president Michelle O'Neill said he had brought "clarity" to Labour's position.
She suggested that during their meeting Sir Keir acknowledged that Labour had previously not been clear and consistent on the need for a backstop.
"It was important today to put to Keir Starmer that we need to see clarity from the Labour Party on the backstop," she said.
"He did say in the meeting that the backstop is inevitable and that is the Labour position.
"It's their position because they understand the need to ensure there's no hard border on this island."
She added: "The words he used to ourselves in the meeting today were clearly that a backstop is inevitable - that is the Labour position in order to protect or guard against a hard border and I can only say that that's what he's relayed to us."
Ms O'Neill said it was important that Sir Keir heard first hand "very clearly and directly" the need to protect the backstop.
She described the backstop as the "least worst option" but nevertheless she said the Dublin government and the EU needed to stand firm against the British government's attempts to alter the Withdrawal Agreement.
"The backstop is a legal guarantee," she said.
"There's no-one here who trusts the word of the British government at this moment in time."
She added: "It has to be legal guarantees. The backstop is the bottom line."
