Tory leadership front runner Boris Johnson has refused to commit to reduce net immigration after Brexit as he faced his rival in the final head-to-head debate of the contest.

The former foreign secretary and Jeremy Hunt agreed that the vote to leave the European Union was a message to “control” immigration, but Mr Johnson did not say how he would achieve it.

“I’m not going to get into some numbers game with you,” he said when quizzed on the issue.

His comments are likely to cause concern among some Tory Party members who would like Mr Johnson to bear down on net immigration if he becomes the next prime minister.

Mr Hunt pointed to his time as health secretary to describe how he would bring down numbers.

“It’s boosting the education and skills levels of our own people that’s the right way to do it,” he said.

Dominic Raab, an ally of Mr Johnson, insisted after the debate that the front runner had a “clear plan” for immigration.

He told PA: “He has got a clear plan of what he wants to do with immigration.

“I think given that public trust has been eroded in the target that we have had and failed to meet, it is right to be just a bit careful about setting a precise target in the middle of a leadership contest debate.”

The pair vying to be the next prime minister also both declined to label US president Donald Trump racist for telling four congresswomen of colour to go back to their “broken and crime infested” countries.

Elsewhere, the Guardian reported that Mr Johnson argued that Islam has caused the Muslim world to be “literally centuries behind” the West, in an essay a decade ago.

Tory leadership race
Boris Johnson during the debate (Louis Wood/The Sun)

In an appendix added to a later edition of his 2006 book on the Roman empire, Mr Johnson reportedly wrote: “There must be something about Islam that indeed helps to explain why there was no rise of the bourgeoisie, no liberal capitalism and therefore no spread of democracy in the Muslim world.

“It is extraordinary to think that under the Roman/Byzantine empire, the city of Constantinople kept the candle of learning alight for a thousand years, and that under Ottoman rule, the first printing press was not seen in Istanbul until the middle of the nineteenth century. Something caused them to be literally centuries behind.”

Mr Johnson was reminded by the host of the head-to-head debate about a 1998 column in which he wrote about “tank-topped bum boys in the Ministry of Sound”.

He replied: “If you are going to excavate and disinter every single quotation from the millions of words I have (written), you can of course twist things one way or the other.”

Tory members are currently voting for their next leader, with the victor to be announced on July 23.

The following day they will be sworn in as prime minister.