Keir Starmer could throw open Britain's borders to the European Union with a youth mobility scheme that is effectively 'unlimited'.

Sources close to talks on the scheme said British negotiators will try to cap EU arrivals allowed to work in Britain at between 70,000 and 80,000, and no higher than 100,000.

But a sliding-scale could be introduced that would effectively allow unlimited arrivals if enough Britons head for the Continent.

One source explained: 'If you suddenly find 500,000 Britons go to Europe, then Britain could open it up to a much larger number of Europeans, so you keep it neutral on immigration figures.'

Another said it could 'essentially be unlimited' if there was enough demand from UK travellers.