Prime Minister Theresa May's Northern Irish allies said Tuesday they would continue to oppose her Brexit deal and raised the prospect of delaying Britain's EU exit by one year to allow more time for negotiations.

"We will not vote for an unamended or unchanged version," the Democratic Unionist Party's Brexit spokesman Sammy Wilson wrote in a commentary in the Daily Telegraph newspaper.

The deal has already been overwhelmingly rejected twice in the House of Commons.

The announcement by the DUP is a blow for May, who earlier had seen the prospects of approval of the deal in a possible third attempt improve when Brexit hardliner Jacob Rees-Mogg declared on Twitter: "The choice seems to be Mrs May's deal or no Brexit".

Rees-Mogg, a conservative MP, heads the European Research Group of eurosceptic lawmakers. The group had so far voted against the deal.

The group is opposed to a long extension to the Brexit deadline because it fears that could lead to no Brexit.

But in reaction Wilson wrote: "There are some colleagues who I admire greatly and who have stood firmly with us in defending Northern Ireland who now take the view that the withdrawal agreement, even though it is a rotten deal, is better than losing Brexit. To them I say that, if the deal goes through, we have lost our right to leave the EU. If we sign up to it, we give away our right to leave to the whim and dictates of the EU. That is not Brexit."