The British government cannot allow the process of opening formal negotiations for Britain to leave the European Union to "drag on", Boris Johnson has warned.

The foreign secretary acknowledged there was a "lot of work" to be done before ministers were ready to invoke Article 50 of the EU treaties - marking the start of the formal Brexit process.

However, he indicated that he would like to see the negotiations - which can take up to two years - wrapped up before the next set of elections to the European Parliament in May 2019.

"There is obviously Euro elections coming down the track," he told BBC1's The Andrew Marr Show.

"I think 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."

Mr Johnson refused to repeat his claim made last week that the government would trigger Article 50 in the early part of 2017 after Downing Street pointedly refused to endorse it.

However, he made clear that Prime Minister Theresa May could not afford to delay for long after the start of the new year.

"We have got a lot of work to get our ducks in order. That is going on. After that, as the prime minister has rightly said, this process probably shouldn't drag on," he said.

"I think what everybody in this country in the UK wants is clarity and getting on with it. Not letting the process drag on is the key phrase I would use."

EU withdrawal will be a marathon not a sprint, experts warn

Civil servants preparing for negotiations to pull Britain out of the EU face a task of "mind-boggling" complexity, experts have said.

With the 100-day mark to be passed later this week since the UK voted to leave, there are signs of impatience among some Brexiteers at the slow pace of progress, with former Ukip leader Nigel Farage warning against "backsliding".

But analysts have said it was likely that Government officials have still not fully quantified the scale of the task ahead, which is likely to stretch well beyond the two-year deadline set under Article 50 of the EU treaties.

Stephen Booth, co-director of the Open Europe think-tank, said it was likely that Brexit would end up being a gradual withdrawal from different aspects of the UK's entanglements with the EU, rather than a single "big bang" event.

The Institute for Government's Brexit lead Hannah White said officials should be working on the assumption that finding a new trade relationship with the remaining EU will take more than two years, and push from an early stage for an interim deal to keep the wheels of trade turning in the transitional period following the end of the Article 50 talks.