Under EU rules, if negotiations are not completed two years after Article 50 is triggered, Britain would simply crash out of the bloc and adopt World Trade Organisation rules, meaning that tariffs have to be imposed on trade between the UK and the EU.

This would be seen as the "hardest" possible Brexit, with no access to the single market.

However, some senior ministers believe this is actually the best-possible solution for Britain because it will allow the UK to be "totally in control of its destiny".

Many Conservatives are pushing Mrs May to adopt a "soft" Brexit, allowing the UK to maintain links to Brussels.