The prime minister is to promise Britain will become a "more prosperous" country after Brexit.

Her claim that the UK will be wealthier is at odds with warnings from a number of independent forecasters. The Office for Budget Responsibility has warned Brexit will "reduce growth in exports and imports" and has revised down growth forecasts for the economy.

But in a speech later, Mrs May will seek to strike a more optimistic note, predicting "a brighter future" after Brexit.

"The promise of Brexit is great, the opportunities before us are enormous," she will say, describing Brexit as "a moment of great national change" which is "alive with possibilities".

She will add: "I am confident we can fullfil the promise of Brexit and build a Britain that is stronger, fairer and more prosperous than it is today."

Labour leader Jeremy Corbyn will also set out his approach to the Brexit negotiations later. Opposition parties have repeatedly warned of the economic risks the UK faces outside the single market.