Theresa May has said she cannot guarantee the status of more than a million UK nationals living in EU countries if Britain fails to reach a deal on Brexit.

Speaking on LBC on Tuesday, the prime minister said she wanted Europeans living in Britain to stay and hoped for a reciprocal deal for British nationals living in the EU.

However, the PM said the rights of 3 million EU nationals in the UK could “fall away” without and exit agreement, and admitted, "we don't know what would happen to” British citizens living in the EU.

“The EU member states would have to consider what their approach would be to those UK citizens,” she told a phone-in caller.

"By definition, if there isn't a deal we won't have been able to agree with the EU what happens to UK citizens currently living in countries like Spain and Italy and other members of the EU."

May said on Monday her Government is preparing to exit the Union without a deal should they fail to reach an accord before March 2019.

“We have teams of people working on every possible outcome,” she said.

“You would expect the Government to prepare because we don't know what will happen. We are working really hard to get a really good deal."

In a September speech in Florence, Italy, May said Britain and the EU were "close" to agreement on citizens' rights.

"I want EU citizens to stay here in the UK, I want to be able to guarantee these rights, we are not going to be throwing EU citizens who are here in the UK out of the UK in the future," she told LBC.

Responding to May’s comments, Labour leader Jeremy Corbyn posted a tweet demanding the PM guarantee “EU migrants' rights now."

Liberal Democrat Brexit spokesman Tom Brake called for greater “clarity” from May.

“Her inability to give a straight answer, her inability to stand up for the rights of our family members, friends, colleagues and neighbours, is damning,” Brake said.

"She is throwing these people under the bus in a bid to appease the right wingers in the Conservative party.”