Facing some of the fiercest attacks to date over her Brexit plans after again failing to clinch a deal at an EU summit last week, May has tried to calm passions in parliament, where her strategy has angered eurosceptics and EU supporters alike.

"Serving our national interest will demand that we hold our nerve through these last stages of the negotiations, the hardest part of all," May told a combative session of parliament on Monday.

Financial markets seized on the possibility that May could be toppled as prime minister by rebels in her Conservative Party, driving sterling below $US1.30 ($A1.83) to its lowest since October 4.

With just over five months until Britain is scheduled to exit the EU, talks have stalled over a disagreement on the so-called Northern Irish "backstop", an insurance policy to ensure there will be no return to a hard border on the island of Ireland if a future trading relationship is not agreed in time.

But May's attempt to unlock the talks by considering an extension to a status-quo transition period beyond the current proposed end date of December 2020 has further riled both pro- and anti-EU factions in her deeply split Conservative Party.

On Monday, in an attempt to highlight how much progress has been made in more than a year of talks with the EU, she told parliament the government has reached agreement on everything from Gibraltar to future security over the last three weeks.

"Taking all of this together, 95 per cent of the Withdrawal Agreement and its protocols are now settled," she said.

"The shape of the deal across the vast majority of the Withdrawal Agreement is now clear."

But the deal - the terms of Britain's divorce - cannot be signed off until the two sides settle on future management of the border between Northern Ireland and EU member state Ireland.

Australian Associated Press