Image copyright PA Image caption Boris Johnson said leaving the EU would be Britain's "independence day"

Boris Johnson only campaigned to leave the EU to set himself up as the next Conservative leader, Sir Alan Duncan said the day before June's referendum.

Sir Alan said he believed the now foreign secretary, who is his current boss, wanted to lose narrowly and be the "heir apparent" to David Cameron.

The foreign minister's comments were made in a BBC Two documentary.

Meanwhile Mr Johnson has told the BBC the formal process of leaving the EU would "probably" begin early in 2017.

The UK voted by 52% to 48% to end its membership of the bloc, in a referendum on 23 June.

But formal negotiations over the withdrawal cannot begin until it triggers Article 50 of the Lisbon Treaty.

Mr Johnson, speaking in New York, said: "The government is working towards an Article 50 letter, which as you know will be produced, probably, in the early part of next year."

'Total chaos'

The process is expected to take two years.

Mr Johnson, repeating his remark to Sky News, added: "In that letter, I am sure we will be setting out some parameters for how we propose to take this forward.

"I don't think we will actually necessarily need to spend a full two years, but let's see how we go."

PM Theresa May has said she will not trigger Article 50 before the end of 2016.

In a behind-the-scenes documentary, "Brexit: A Very British Coup?", Sir Alan - then a backbench MP but now a minister in Mr Johnson's Foreign Office - predicted the result would be 52% for Remain and 48% for Leave.

And he questioned the motives of Mr Johnson in backing the campaign to exit the EU.

"I think there are a lot of Leave people who don't believe it, and I've always thought that Boris's wish was to lose by one so that he could be the heir apparent without having to have all the... you know... of clearing up all the mess, that's always been my view of Boris."

He added: "By championing leave, he can be the great heir apparent of the future, darling of the activists, but actually it would be quite good if he didn't actually win the referendum because there would be total chaos."

Image copyright Getty Images Image caption Sir Alan Duncan is now a minister in the Foreign Office, headed by Boris Johnson

Speaking after the result was known, Sir Alan warned that the UK was entering "a period of deep instability and uncertainty" and "potentially ultra-dysfunctional government"

"And you know, the first thing we hear is Michael Gove says he is going to negotiate with David Cameron about being in charge of the negotiations. Well these people have got to remember they might have won a referendum, but they don't run the country."

Mr Johnson, popular among Tory activists as one of the leading Vote Leave figures, argued that leaving the EU would be Britain's "independence day".

He said the UK could "take back control" - of its money, its immigration system and its democracy and accused Remain campaigners of talking the country down.

'Global player'

After the referendum and David Cameron's ensuing resignation Mr Johnson unexpectedly withdrew from the race to be the next Tory leader and PM, positions he is long thought to have harboured desires for.

He said he did not believe he could provide the leadership or unity needed, after then Justice Secretary and fellow Brexit campaigner Michael Gove made his own bid for the job.

But in a shock move just over a week later - with Mr Johnson's fortunes seen to be at their lowest ebb - Mr Cameron's successor, Theresa May, appointed him as foreign secretary in her new cabinet.

In that role, he has stressed that Brexit does not mean Britain will be leaving Europe, just "leaving the EU", and said he wanted the UK to be a "great global player".

Asked about Mr Duncan's comments by Channel 4 News while in New York for a UN meeting, Mr Johnson said: "I think what's fascinating about being here is... how few people actually want to discuss Brexit, they want to discuss what Britain is doing around the world."

But he added that he believed Brexit was "an opportunity to get out from, I think, the unnecessary burdens of the European Union treaties and do a global free trade deal."

The documentary, Brexit: A Very British Coup?, will be broadcast on Thursday 22 September at 21:00 BST.