The UK government will fail to meet its "tens of thousands" net migration target if Theresa May delivers a so called "soft Brexit", a right-leaning think tank warned on Thursday (29 December).
Migration Watch said that full access to the EU's single market would be incompatible with the Conservatives' pledge to cut immigration since the UK would have to recommit to free movement rules.
The group estimated that net migration from EU citizens to the UK would be "unlikely to fall" below 155,000 in the medium term, compared to more than 189,000 in the year to June 2016.
Migration Watch also predicted that EU citizens would continue seeking work in the UK because of southern Europe's high levels of youth unemployment. Italy and Spain, for instance, have rates of more than 35%.
EU chiefs, such as European Council President Donald Tusk, have consistently ruled out what they dub "single market a la carte" for the UK – full access to the economic bloc and immigration curbs.
John Bickley, Ukip's immigration spokesman, said: "May needs to set out in no uncertain terms that when the UK leaves the EU it will also cease to be a member of the single market.
"The UK will then, like the rest of the world have access to the single market, and tariff free access will be hugely beneficial to the EU, who run a massive trade surplus with the UK."
But Liberal Democrat leader Tim Farron warned that "withdrawing" from the EU's single-market would risk thousands of jobs. "It's no surprise that Ukip's favourite think tank is pushing for a hard Brexit that would rip Britain out of the single market, costing jobs and risking the livelihoods of thousands of British people," he said.
"A recent opinion poll showed that 90% of the British public want to remain in the single market. They do not want a hard Brexit that would leave Britain economically isolated and poorer.
"The Liberal Democrats are proud to be the party of common sense, providing the real opposition to the Conservative Brexit government by fighting to remain in the world's most lucrative market."
May has promised to invoke Article 50, the mechanism to split from the EU, by the end of March 2017.
Her government has also promised to publish a Brexit "plan", which Brexit Secretary David Davis said he did not expect to be released before February.