Theresa May's handling of the border question could bring Brexit negotiations "crashing down", according to the UK Government's former chief negotiator on Northern Ireland.
Jonathan Powell said May has committed "the worst possible sin a negotiator can commit" - having "boxed herself in", in an article for The Independent.
It comes as the EU and the UK said no agreement had been reached on the approach to avoid a hard border on the frontier.
Mr Powell the British Government's proposal of enjoying "the benefits of the single market in the areas we choose without taking on the obligations it requires" is at the centre of the problem.
Tony Blair's former chief of staff warned that a return to "huge concrete slabs" and "checkpoints on the main roads" could "force Northern Ireland back into identity politics".
The former diplomat wrote: "She will be faced by the alternative of accepting the current EU draft and losing the support of the 10 DUP MPs who sustain her Government, or alternatively accepting that the whole of the UK remains in the single market and the customs union, in which case she will lose the support of the 62 backbench Brexit Tories."
He argued that Mrs May had been able to "fudge" the issue until now but that strategy "doesn't work any more".
Mr Powell added: "Theresa May's speech may have temporarily united the Tory party, but it has not solved the substantive problems.
"Indeed, her problems on Brexit may only just have begun and it may turn out that the insoluble problem of the Northern Ireland border is the issue that finally brings the entire negotiation crashing down."