LONDON (AFP) - Britain should not let its European Union exit talks "drag on", Foreign Secretary Boris Johnson said Sunday (Sept 25), though he refused to repeat his previous claim the government would trigger the formal leaving process in early 2017.

Mr Johnson said on Thursday that Britain would invoke Article 50, the official procedure for quitting the European Union, within months of the new year.

But he was not subsequently backed up by Prime Minister Theresa May's Downing Street office, which repeated her position that the provision would not be triggered this year.

Mr Johnson, who spearheaded the campaign to leave the EU in Britain's June referendum, said on Sunday that Britain should exit before the next European Parliament elections in May 2019.

"People will be wondering whether we want to be sending a fresh batch of UK Euro MPs to that institution which, after all, we are going to be leaving. So let's get on with it," he told BBC television.

He was pressed three times about the timing but declined to repeat his previous assertion.

"Obviously we are not going to do it (trigger Article 50) before Christmas and I think we've got to do a lot of work to get our ducks in order and that is going on.

"But then after that, as the prime minister has rightly said, this process probably shouldn't drag on."

Part of the negotiations will involve Britain's level of access to the European single market and whether it imposes controls on immigration from EU countries.

Mr Johnson also called for greater investment in youth skills and training.

"For 25 years UK business and industry have been mainlining immigration like a kind of drug," he said.

Meanwhile, former prime minister David Cameron's communications chief said Mr Johnson could not make up his mind whether to back Brexit or not in the June 23 referendum, sending Mr Cameron conflicting texts in quick succession.

"I am struck by two things: Boris is genuinely in turmoil, flip-flopping within a matter of hours; and his cavalier approach," Mr Craig Oliver said in his memoirs, serialised in The Mail on Sunday newspaper.

After plumping for the Leave campaign, Mr Johnson told Mr Cameron he expected to be on the losing side as Brexit would be "crushed".

Mr Cameron "says Boris is really a 'confused Inner', and their previous conversations confirmed that view to him", Mr Oliver wrote.