"You can regard it as binding," the spokeswoman said when asked about the vote announced earlier in the day by May, which will come at the end of a negotiating period with the EU.

Asked what a 'No' vote by lawmakers would mean she said "We will be leaving the EU."

Asked whether a 'No' vote would mean Britain would then be relying upon World Trade Organisation terms, she said "Either way, we will very clearly be leaving the EU."

(Reporting by William James; editing by Stephen Addison)