CNN  — 

British Prime Minister Theresa May was dealt another humiliating blow Thursday as lawmakers voted against the latest incarnation of her Brexit plan.

While the vote was merely symbolic, Parliament’s refusal to reaffirm its support for May’s approach to negotiations with the European Union represented another setback for the government as it attempts to reopen the terms of its Brexit deal with the EU.

Lawmakers voted 303 to 258 against the government’s motion, which included an amendment on renegotiating the Irish backstop – a contentious part of the deal – along with a nonbinding amendment to prevent a no-deal Brexit.

A group of hard-line Brexiteers in May’s Conservative party abstained from the vote on Thursday, unwilling to back a motion that aimed to block Britain from crashing out of Europe on March 29 without a deal.

May has previously refused to rule out a no-deal Brexit, even though a majority of lawmakers say it will plunge the UK into chaos. That is with the exception of Brexiteers, who maintain that keeping it on the table is a necessary negotiating tactic with the EU.

The rebellion by members of her own party will further undermine May’s pledge to the EU that if it renegotiates its deal with Britain, she can get a revised agreement approved by Parliament.

“Tonight’s vote shows there is no majority for the Prime Minister’s course of action in dealing with Brexit,” opposition leader Jeremy Corbyn said, adding that he was surprised May did not stick around to answer questions after the vote.

“She cannot just keep on just running down the clock and hoping that something will turn up that will save her day and save her face,” Corbyn added.

A Downing Street spokesman countered that Corbyn had made a no-deal Brexit more likely by voting against the government’s motion.

“The Prime Minister continues to believe, and the debate itself indicated, that far from objecting to securing changes to the backstop that will allow us to leave with a deal, there was a concern from some Conservative colleagues about taking no deal off the table at this stage,” the spokesman said in a statement.

And while May will press ahead with her plan to secure changes to the backstop – an insurance policy designed to avoid a hard border between Northern Ireland and the Republic of Ireland – those negotiations are likely to be complicated by Thursday’s vote.

EU diplomats dealing with Brexit told CNN they didn’t need the vote to tell them May doesn’t command a majority in Parliament, but that the latest defeat had underlined that reality.

There is growing frustration in Brussels with May, who is seen as a less reliable negotiating partner by the day.

“She’s trying appeasement but it’s just not working. You would think there should be a moment she has to sacrifice party unity to ensure safe landing for her country. But [I] doubt she will, hoping eventually the EU moves in the end,” one EU diplomat told CNN.

Ireland’s Deputy Prime Minister Simon Coveney said Thursday night that it was Britain’s responsibility to break the current impasse – not the EU.

“We should say very firmly that the responsibility to come up with ways of solving the current impasse lies where the impasse is, which London – not Brussels, not Dublin, not Belfast or anywhere else,” Coveney told the Irish Parliamentary Foreign Affairs Committee.

CNN’s Erin McLaughlin contributed to this report from Brussels.