Britain's Prime Minister Theresa May speaks during a debate on her Brexit 'plan B' in Parliament. File picture: Mark Duffy/UK Parliament.

London - Opposition lawmakers accused Prime Minister Theresa May of stalling on Tuesday after she asked for more time to negotiate changes to a "backstop" protocol to protect an open Irish border after Britain leaves the European Union.

May said she had told EU leaders what parliament wants "in order to unite behind a withdrawal agreement: namely, legally binding changes to the backstop."

"Having secured an agreement with the European Union for further talks, we now need some time to complete that process," she told parliament's main elected house, the Commons.

Labour's Jeremy Corbyn, who leads the biggest opposition party, accused May of introducing "more excuses and more delays," less than seven weeks until Britain is scheduled to leave the EU on March 29.

"It appears the prime minister has just one real tactic: to run down the clock hoping members of this house are blackmailed into supporting a deeply flawed deal," he said.

"She is playing for time and playing with people's jobs, our economic security and the future of our industry."

May suffered a crushing defeat in parliament when the draft Brexit deal was put to a vote in mid-January.

"When we achieve the progress we need, we will bring forward another meaningful vote," she said on Tuesday.

She promised to make another statement to parliament on February 26, with a vote on what should happen next the following day, if the Brexit deal has still not been finalized.

"The talks are at a crucial stage," May said. "We now all need to hold our nerve to get the changes this house has required and deliver Brexit on time."

"By getting the changes we need to the backstop; by protecting and enhancing workers' rights and environmental protections; and by enhancing the role of parliament in the next phase of negotiations I believe we can reach a deal that this house can support," she said.

Ian Blackford, the Scottish National Party (SNP) leader in the British parliament, told May she was "lost in a Brexit fantasy" since EU leaders have repeatedly ruled out reopening the withdrawal agreement, including the backstop.

Anna Soubry, a leading pro-EU lawmaker in May's Conservatives, said her party leader's statement was "unacceptable."

"Parliament is in an impasse so let's get this back to the people," tweeted Soubry, a leading campaigner for a second referendum on Brexit.

May has said she wants to change the backstop arrangement to prevent the creation of a "hard" border between Northern Ireland and the Republic of Ireland after Brexit.

A hard border on the island of Ireland - including greater customs and immigration controls - would breach the UN-ratified Good Friday Agreement which brought an end to decades of political violence in Northern Ireland.