Before the EU referendum, Boris Johnson was the bombastic, outspoken, charismatic Tory who had governed a Labour-leaning capital city for eight years. Like political honey he had a unique ability to entice non-Conservative voters to jump ship.

Then on 21 February 2016 the former London mayor came out in favour of Britain leaving the EU – and almost overnight he was transformed into political Marmite.

By the start of this year’s snap election campaign, Conservative party internal research, carried out by Sir Lynton Crosby’s company, painted a clear picture.

Johnson had become a divisive figure. Yes, he had the power to lure in leave voters, often in traditionally working class, Labour areas. But elsewhere he was seen as a toxic figure. And if there is one thing that lights the touchpaper among voters already suspicious of figures such as Johnson, it is the key promise that many believe helped deliver the Brexit vote – £350m a week for the NHS.

Q&A What was wrong with the claim that the UK sends the EU £350m a week? Show The claim that Britain “sends the EU £350m a week” is wrong because: The rebate negotiated by Margaret Thatcher is removed before anything is paid ​​to Brussels. In 2014, this meant Britain actually “sent” £276m a week to Brussels; in 2016, the figure was £252m.

Slightly less than half that sum – the money that Britain does send to the EU – either comes back to the UK to be spent mainly on agriculture, regional aid, research and community projects, or gets counted towards ​the country’s international aid target. Regardless of how much the UK “saves” by leaving the EU, the claim that a future government would be able to spend it on the NHS is highly misleading because: It assumes the government would choose to spend on the NHS the money it currently gets back from the EU (£115m a week in 2014), thus cutting f​unding for​ agriculture, regional development and research by that amount.



It assumes​ the UK economy will not be adversely affected by Brexit, which many economists doubt.

All of which helps explain why the foreign secretary may have revisited this particular claim – to defend himself against what he considers unjust attacks.

And the spitting anger in his letter to Sir David Norgrove, the head of the UK Statistics Authority, underlines his strength of feeling, as he believes Britain will ultimately regain control of that amount and be able to spend much of it on the NHS.

But the motivation and timing of his 4,200-word article published on Friday is clearly more complicated than that. Johnson is the torchbearer of Brexit. And on Friday, Theresa May is to deliver a speech that is rumoured to strike a conciliatory tone and perhaps the promise of continued payments after leaving the EU.

It is that suggestion that motivated the foreign secretary to write. Mindful of that £350m promise (which cabinet colleagues have reminded him was a Vote Leave pledge, not theirs) Johnson knows he has a reputation to protect.

Yet, despite the widespread interpretation of the piece, Boris is not understood to be against payments during transition (although he wants a significantly shorter implementation period than is likely, one of just six months to a year). However, he feels strongly that money should not be paid after that point.

Johnson knows that after the powerful Vote Leave promise of a long-term boost to public services, ongoing divorce payments would be unacceptable to leave voters. In that respect, Johnson’s piece does represent an undermining of May’s authority just days before she is due to deliver the most important speech of her premiership.

Not that the prime minister can do anything about it. As one former senior Downing Street adviser told the Guardian – with a minority government propped up by the DUP, May can’t afford to upset the careful equilibrium she pieced together after the devastating general election result.

Besides, Johnson’s piece is a long, thoughtfully argued vision for Brexit that is at no point explicitly disloyal. And yet this journalist turned politician should hardly be surprised by the fuss the article has caused.

Firstly, it was unquestionably poor timing to be published on the night of what could have been a major terrorist attack on British soil. And, secondly, the verbal outpouring of a man known to have his eye on the party leadership was always going to be scrutinised for any phrase or even omission (not mentioning transition) that could be seen as an act of disloyalty.

The result was always going to be seen as a firing shot for future leadership hopes – although few in the Tory party want to see May’s government destabilised too quickly lest it provide an opportunity for Jeremy Corbyn.

Downing Street, meanwhile, suggested listening to Amber Rudd’s “back seat driving” riposte to Johnson. And the fact that Michael Gove has also distanced himself from the foreign secretary on at least some points has left him somewhat isolated.

Johnson may have made his point, but the inevitable backlash means this intervention will do little to chip away at the love-hate persona, which is likely to be his biggest challenge if he ever does lead the Tory party.