"It is not perfect," she said during the parliamentary debate. "But when the history books are written, people will look at the decision of this house and ask, 'Did we deliver on the country's vote to leave the EU, did we safeguard our economy, security, or union, or did we let the British people down?'"

British voters decided in a 2016 referendum to leave the EU and the nation has been divided ever since on whether that narrow decision -by 52 percent to 48 percent-was right and, if it was, whether the UK should leave but maintain close ties with the EU, or make a clean break.

With such divergent views, May has struggled to build a consensus.

Tuesday's Brexit debate was set to end around 7 pm and MPs were expected to follow it by voting on several proposed amendments before settling down for the main vote, which was expected between 8 and 9 pm.