Sir Keir Starmer has said the UK Government must be held to "solemn commitments" to avoid a hard border in Northern Ireland after Brexit.
The Shadow Brexit Secretary made the comment after meeting business leaders and traders from both sides of the border during visits to Belfast and Londonderry on Monday.
Sir Keir said there must be "no rowing back" on the commitments made as the first round of Brexit negotiations opened last December.
He said: "Obviously one of the main talking points has been how do we hold everybody to the really solemn commitments that were made in phase one to no hard border in Northern Ireland, meaning no infrastructure, no checks and no controls.
"Even just looking at the border, as we were doing, it is hard to imagine how that could possibly change and the solemn commitment is, that it won't change.
"I think it is really important that the Government is held to that."
Labour wants the UK to remain in the single market and the customs union during the transitional period.
Sir Keir added: "No-one, but nobody wants anything by way of checks and infrastructure on the border, nor should they.
"The only way that is going to be possible is if the Government accepts the red lines they set down, sweeping options such as single market and customs union off the table, have got to go.
"Labour has been saying for some time now that we need transitional arrangements on the same basic terms as now and that means in a customs union in the single market.
"The customs union should always have been a viable option, the Government should never have swept that off the table."
In December, it was agreed to protect north/south co-operations with no regulatory divergence between Northern Ireland and the Republic of Ireland across a range of key industries.
Sir Keir said bitter divisions within the Conservative Party were now the biggest threat to securing a deal.
He said: "Now the single biggest threat to a progressive, collaborative partnership with the EU is the in-fighting in the Government and so we have been here trying to have the conversations that are necessary to ensure that we, as the opposition, can offer a proper alternative to what the Government is doing."
In addition to travelling to the border, Sir Keir also met political representatives involved in the latest round of negotiations to save the devolved Stormont institutions.
Speaking before the meeting, SDLP MLA Claire Hanna said: "I believe him that he doesn't want to see a hard border but clearly, so far down the line since the referendum, it is very clear that the only way to do that is to maintain full single market access for the entirety of the island."
During a separate meeting, Sinn Fein's Michelle O'Neill MLA urged Sir Keir to help secure designated special status for Northern Ireland.
She said: "I reiterated to Keir Starmer that Brexit will be an disaster for the entire island of Ireland, both socially and economically.
"I made clear that as negotiations move on to phase two that we cannot withstand exclusion from the single market and customs union. Otherwise the reality is economic apartheid on this island as the result of a hard border.
"I urged him to help ensure that the British Labour Party throws its weight behind the argument for special status for the North within the EU and to help secure it in the best interests of all citizens here."
Meanwhile, Ms Hanna also expressed frustration that round-table discussions involving all five of the main Stormont parties had not yet taken place.
She said: "I think everybody is frustrated. It is over a year that we have been in this position. There is some hope if compromise was reached and was being talked about and compromise isn't always being seen as a dirty word.
"We can't negotiate with one arm behind our back without having access to that information and we certainly believe that the public deserve to know what progress has or hasn't been made and what stretching of themselves people have or haven't done."