Former Tory MPs Douglas Carswell and Sir Malcolm Rifkind have crossed swords over Theresa May's leadership campaign

And speaking at the Centre for European Reform's Brexit conference today, Carswell, who has represented Ukip in parliament since 2014, said the home secretary's views were “regrettable”.

“There are about 3m EU nationals living in this country, and it must be made absolutely clear that their right to remain here is non-negotiable.

“I personally think its very regrettable that one of the candidates for the leadership of the Conservative party has been a little bit iffy about this,” he said

“I think they need to provide that reassurance and provide it straight away. People living in this country have every right to remain in this country, and that is not negotiable.”

However, May was backed by Rifkind, who argued that as the favourite to succeed David Cameron, the home secretary has erred towards caution, but had nonetheless been prepared to offer some support.

“Because Theresa may know she is going to be to prime minister and the others don't expect to be, she's had to be slightly more cautious in the language she has used,” Rifkind said

“I think the formula she is using is the right one. She has said, I believe in the right of EU nationals I to remain in the United Kingdom as long as there is reciprocity for British nationals elsewhere in the European Union.

“She has to use careful language, but its quite clear that's what we will end up with.”

Separately, Rifkind also warned that the loss of foreign direct investment represents “the most serious economic disadvantage” of the Brexit vote.

“I can't see any company from other countries in the world choosing London on the UK in the short to medium term if it wants to have access to the European market,” he said.

“I don't think we're going to lose all these companies, but we're going to lose significant parts of their current UK presence, as they move HQs or other tasks to Europe.”