BELFAST (Reuters) - The Northern Irish party propping up British Prime Minister Theresa May’s government on Saturday said it would vote against her Brexit withdrawal agreement, which its deputy leader said would leave Britain in a “pitiful and pathetic place”.

The pledge to vote down the deal was made by party leader Arlene Foster at the end of her Democratic Unionist Party’s annual conference at which members gave standing ovation to key May rival Boris Johnson’s proposal to begin preparations for a “no deal” Brexit.

“We will vote against it if its brought to parliament,” Foster told Ireland’s RTE television in an interview from the conference, in her most direct comments on the issue to date.

May’s divorce treaty and accompanying political declaration are due to be endorsed in Brussels on Sunday by May and the other 27 EU leaders.

But Foster’s comments further complicate May’s already daunting challenge to get enough votes to pass the deal in parliament.

Dozens of members of her own party have indicated they will not support the deal and the opposition Labour party plans to vote it down.

While the DUP’s 10 members of parliament are subject to a ‘confidence and supply’ deal signed last year last year to back her minority government, Foster said they would not vote with the government and would consider scrapping the deal if the agreement somehow passes.

Slideshow ( 4 images )

The withdrawal treaty’s ‘backstop’ provision could align Northern Ireland more closely with the EU than the rest of the United Kingdom if no other way can be found to avoid a hard border with the Republic of Ireland.

Foster’s pro-British party fears the provision, if implemented, could one day threaten the province’s place in the UK altogether.

The backstop “will put Northern Ireland under swathes of EU laws with no say for anyone in Belfast or London (and create) a trade border down the Irish Sea,” deputy DUP leader Nigel Dodds told the conference.

“The published Withdrawal Agreement portrays a pitiful and pathetic place for the United Kingdom,” he added, saying it would be “locked into an EU straitjacket, divided and diminished.”

The comments were echoed by Johnson, who received a rapturous applause from the delegates, saying Britain would be left as a “satellite state” by the deal.

May has said the EU will not renegotiate and that Britain could face a hugely disruptive “no deal” Brexit if it does not take the deal on offer.

Slideshow ( 4 images )

Foster said to regain the DUP’s support May would have to remove the Irish protocol from the legally binding withdrawal agreement, something the EU has repeatedly made clear it will not countenance.

The Irish border issue should be in the the non-binding political declaration instead, she said.

If parliament votes down May’s deal that will trigger renewed negotiations rather than an election or another referendum, she said.

Earlier Foster told the Times newspaper that a government led by the Labour Party’s Jeremy Corbyn - a longtime sympathiser with the DUP’s arch rival Sinn Fein - might be preferable to the deal.

(This story has been refiled to correct additional reporter’s name in signoff.)