Britons may have to apply for visas to travel throughout continental Europe once the UK leaves the EU, it has been reported.
A scheme apparently being debated by the executive body of the European Union suggests the 26-nation passport-free Schengen zone, which does not include the UK, could operate a visa programme similar to the US waiver.
Currently British passport holders can travel throughout member states without having to apply for short-term visas, but Britain's decision to leave the EU has left question marks over the criteria needed for UK nationals to visit the Schengen zone.
According to The Guardian, the European Commission (EC) is due to unveil draft legislation for the EU travel information and authorisation system (Etias) later this year as part of a broader response to calls for greater security across the continent following recent terror attacks in France and Belgium.
The scheme would cover all visitors to the Schengen zone from countries that do not need a visa to enter.
Camino Mortera-Martinez, a research fellow specialising in justice and home affairs at the Centre for European Reform, quoted in the newspaper, said: "In theory UK citizens, as third-country nationals, would certainly be subject to the obligations (of such a scheme)," adding: "This will have to be part of the Brexit talks. It will all have to be negotiated."
Liberal Democrat leader Tim Farron said: "Theresa May has said ending freedom of movement was her priority, but presumably this was not what she had in mind.
"This week the Liberal Democrats laid out a plan about what the Government should prioritise in any Brexit negotiation. One of those priorities was travel rights.
"The British Government needs to drop the empty mantras and secure free travel for British holiday and business travellers.
"Liam Fox, Boris Johnson and David Davis simply aren't getting their heads round the hard detail of Brexit negotiations. The Brexit Towers trio are saying a lot, just not doing a lot."
Labour former minister Pat McFadden, of the Open Britain campaign, said: "Reality is setting in. Introducing visas would hurt British businesses and families going on holiday.
"The Leave camp repeatedly said visas wouldn't be introduced but it now looks like another of their main promises is being broken.
"Britain should be open to business, travel and talent and the best way to do that is for us to be a member of the single market."