Boris Johnson, the frontrunner to be Britain's next prime minister, said Saturday that he supported the idea of an amnesty for illegal migrants.

However, he said any efforts to regularise the status of long-term irregular workers must be matched by a clampdown on new arrivals to avoid creating a "pull factor"

Johnson first floated the idea of an amnesty when he was mayor of London. Now running for leader of the ruling Conservatives, he said he remained in favour.

He suggested there were around 500,000 people in London who had lived there for a decade or more but never registered and "are not able to pay taxes".

"I don't think it's commonsensical to think we can deport such a large number of people. We do need to think of how to regularise their status," he told a leadership campaign event in the east of England.

He said that despite the government's efforts to clamp down on irregular migration, deporting people was "very difficult legally" and the number of removals was "vanishingly small".

"What I'm proposing would probably not make much practical difference in the existing state of affairs, it would regularise what is already going on."

However, he added: "We have to show that we would simultaneously deport first bounce, as it were, people who are coming in illegally."

Johnson led the campaign to leave the European Union in 2016, which was dominated by promises to control migration by ending free movement from the bloc.

He himself is a strong advocate of immigration, saying it was one of the reasons London and the south-east of England was so dynamic.

However, he acknowledged many voters wanted to know there was some control, and has previously proposed to ensure new arrivals speak English, have certain skills and have a job offer.

"What we want is a democratically controlled immigration system whereby we can continue to welcome people who can contribute to the UK economy, but done by an Australia-style points system," he said.

Britain was rocked by a scandal last year over the deportation of people who migrated legally with their parents from the Caribbean in the 1970s, but never formally regularised their status.

"On any compassionate, commonsense view of the situation that was the wrong thing to do," Johnson said.