One of the most frequent whispers in Labour circles is that Jeremy Corbyn is not truly in control of the Labour Party. The story is the Westminster equivalent of Japanese knotweed: stealthy, widespread and apparently immune to all attempts to kill it off.

In the two years since Corbyn became Labour leader, staffers, MPs and even shadow ministers have assured me that the “real” Labour leader is, variously: Simon Fletcher, Corbyn’s first chief of staff, since departed of his own volition; Jon Lansman, the chief of Momentum, who is not even Corbyn’s first choice to fill the vacant role of party general secretary; Karie Murphy, who wasn’t even Corbyn’s first choice to be his chief of staff; and John McDonnell, his shadow chancellor.

The rumour has no particular factional slant: outside the immediate inner circle of McDonnell, Diane Abbott, Jon Trickett and their close aides, you can hear it from almost anyone in Labour, regardless of how they feel about the Corbyn project overall.

That it is not true should be obvious from the many of Corbyn’s supposed controllers who have found themselves either dismissed or sidelined. Yet the proof of its falsehood is obvious when you consider the shadow cabinet. Were almost anyone but Corbyn himself in control, Nia Griffith – the shadow defence secretary who openly defied her leader on the campaign trail – or Debbie Abrahams, who is seen as having failed to make an impact in the welfare brief, would be gone. Both are protected by Corbyn’s loyalty to those who “stuck with him” during the 2016 attempted coup.

The rumour endures because it speaks to two essential truths about Corbyn. The first is his natural self-effacement: one relatively junior staffer was surprised when at their first meeting between the leader’s office and the shadow Treasury team Corbyn volunteered to make tea for the attendees.

The second is that, unlike most Labour MPs, Corbyn’s passion is foreign affairs. That means he is happy to defer on domestic matters to McDonnell and Abbott, which can give the impression of subservience.

Yet even that deference is often strategic. Corbyn has been a friend and political ally of both Abbott and McDonnell for three decades. As those who know him well point out, that means he understands where their politics diverge from his. “If Jeremy brings Diane in on, say, Europe, he’s not doing it because he thinks she’s going to call for a hard Brexit,” one well-placed observer of the party’s ruling quartet points out.

Equally, when he turns to Trickett – who, like Corbyn, campaigned for Leave in the 1975 European referendum – the Labour leader isn’t expecting to hear a lengthy address on the grandeur of the European project. Nor does he turn to the former journalist Seumas Milne, whom he has known for less time but holds in high regard, expecting to be told that the EU is a virtuous organisation. No one who knows Corbyn well doubts that he is more instinctively sympathetic to the Milne view of the EU than the Abbott one. During the referendum campaign, he was convinced to back Remain because he saw it as a necessary condition for retaining the Labour leadership.

On a less cynical level, he was also swayed by Yanis Varoufakis, the Greek economist who is well-respected in the leader’s office and by Corbyn in particular. The former Syriza finance minister told him that Brexit would precipitate the collapse of the EU, causing disproportionate suffering to the bloc’s smaller and poorer nations.

This, of course, has not happened. That means Corbyn has felt free to return to his old Euroscepticism, in particular his hostility to the European Court of Justice and what much of his inner circle see as its record of “neoliberal” rulings, such as the Viking Line judgment, which ruled in favour of a large shipping company against an international group of trade unions.

The shift in the party’s position on Brexit – particularly its openness to a customs union with the EU – has given the Corbyn conspiracists another boost.

Has the Labour leader been forced to change tack? And is it a sign of weakness?

On the first question, Corbyn has certainly not repudiated his opinion that the EU, which is dominated by right and centre-right governments, is not naturally congenial to left-wing ideas. The rise of Emmanuel Macron in France and the continuing influence of Angela Merkel in Germany have confirmed this view in the leader’s office.

Indeed, Milne was a forceful advocate of maintaining Labour’s previous stance against a customs union, which was backed up by a Commons vote on 20 November.

Corbyn’s drift away from that position was driven by his recognition of three fundamental truths about the state of British politics. First, the Tory government has a wafer-thin domestic agenda, so the only way to defeat the Conservatives in the Commons is on its Brexit-related bills.

Second, most Labour voters (although not Labour constituencies) backed Remain, and there is also scope for peeling away the third of Tory voters who wanted to stay in the EU. Third, pursuing a softer – and therefore less economically disruptive – Brexit wins him plaudits from business groups, such as the CBI, as well as the trade unions.

“The truth,” as one ally puts it, “is that Jeremy has discovered, in the very final stage of his political career, that he actually wants to be prime minister.” Hence, in his speech in Coventry on 26 February, another unexpected move from Corbyn, a former chair of the Stop the War Coalition: praising Tony Blair for his role in bringing peace to Northern Ireland.

Who’s controlling Jeremy Corbyn? No one. But something is driving him – his desire to lead the country.