Theresa May faces a new Brexit headache as the UK's Labour prepares to forge a cross-party alliance to guarantee there will be no infrastructure at the Northern Ireland border.
The plan would see an attempt to rewrite Brexit legislation to enshrine in law the political commitment made to avoid a hard border.
Such a move would rule out customs checks and border controls at the frontier - forcing the UK Government to come forward with alternative arrangements.
It would also protect the commitment to "no hard border" even if the UK walked away from Brexit talks with Brussels without a deal.
Shadow Brexit Secretary Keir Starmer told the Observer: "At the end of last year, the EU and UK government made a political agreement that there would be no hard border in Northern Ireland.
"However, the content of the withdrawal agreement is not legally binding.
"It is a political document subject to negotiation and will not have legal force unless and until it is ratified - which is by no means a done deal."
The plan could take the form of an amendment to the European Union (Withdrawal) Bill currently going through the House of Lords and Labour would hope to persuade peers from other parties to back it.
That would then force a showdown in the House of Commons when the legislation returns there later this year.