The Brexit spokesman for Britain's opposition Labour Party said it will propose legislation to stop Britain leaving the European Union without a deal if Parliament rejects the final agreement struck with Brussels.

Britain is due to leave the 28-country trading bloc at the end of March 2019. Last week the other EU member states approved guidelines for relations between Britain and Europe during a 21-month post-Brexit transition phase, stretching until December 2020.

British Prime Minister Theresa May's Conservative Party only has a minority government, propped up by Northern Ireland's Ulster Unionist Party, and if no satisfactory parting of the ways can be agreed, Labour says it hopes it can get cross-party backing to defeat a "take it or leave it" approach, whereby any parliamentary rejection of the deal offered is regarded as approval of a "no deal" Brexit.

"Labour will ensure that an amendment is tabled to the EU Withdrawal Bill," said its Brexit spokesman Sir Kier Starmer, in released extracts of an upcoming speech.

"Should the prime minister's deal be defeated, it must be for parliament to say what happens next, not the executive.

"Labour's preference in that scenario is clear: The government should go back to the negotiating table and work toward securing a deal that works for Britain. This would provide a safety valve in the Brexit process."

Brexit secretary David Davis said it is "incredibly probable" a deal will be reached on Britain's departure from the EU, while also defending having a contingency plan for a "no deal" exit as being like having an insurance policy.

The transition deal approved last week has drawn a mixed reaction in Britain, with many business leaders hailing it as a common sense approach to allow adaptation to new circumstances, while pro-Brexit groups and politicians have accused May's government of caving in to European demands.

Former Labour prime minister Tony Blair, who favors a second referendum on the issue, has also joined the debate, calling the approach May's government is taking "deeply, dangerously irresponsible "and warning that her Conservative Party is" in mortal danger of putting a proposition to parliament which will not pass".

He said the "sensible strategic course" is for the Conservatives to "share the responsibility" and to let Parliament decide.