Boris Johnson's father, Stanley, has backed the government's plan to slash immigration numbers as part of its Brexit plan, despite campaigning for a Remain vote at the EU referendum. Johnson, the founder of Environmentalists for Europe, told IBTimesUK that Britain's population was rising at an "unsustainable" rate, while praising David Davis' speech to the Conservative party conference in Birmingham yesterday (2 October).

"Britain's population is rising at something like half a million people a year, that is just unsustainable," Johnson said.

"It's not just immigration, it's also a result of fertility. It has to be done.

"I've written books about population, the background behind all of this is the pressure of people on social services, welfare, on the environment – it's just not sustainable."

Davis' comments were the clearest indication yet that the UK could face a "hard Brexit" and be refused access to the EU's single-market because of its support for immigration reforms.

The free movement of peoples is one of the fundamental principles of the economic union and EU chiefs, including European Council President Donald Tusk, have ruled out a "single-market a la carte". The issue was a major debating point of the referendum campaign, with Vote Leave backing an Australian-style visa system.

David Cameron had promised to cut net migration down to "tens of thousands", a pledge Theresa May's government is sticking with despite official figures from the Office for National Statistics (ONS) showing that levels have consistently hit more than 300,000.

Johnson also told IBTimes UK that May was right to announce that the government would trigger Article 50, the official mechanism to split from the EU, before March 2017.

"It's absolutely the right thing to do this, we need clarity now, I think the prime minister spelled out very clearly what the procedure is going to be and I think she was perfectly right to say we do not need a Commons vote to trigger Article 50, she was perfectly right to rule out a second referendum, so now we go for it," he said.

As for his Foreign Secretary son, Johnson said Boris has "done absolutely fine". He added: "He gave a completely invigorating performance yesterday. There were some very solid messages in there.

"I particularly enjoyed and appreciated the message that actually you can't spectate economic prosperity from political freedom."