A year and five days before the UK is due to leave the European Union, Keir Starmer is confronting a series of uncomfortable truths. He never wanted Brexit to happen and still doesn’t – but he accepts there is nothing he or anyone else can do to stop it. Asked if the impending breach with our 27 EU partners is now inevitable, the shadow Brexit secretary is clear. “Article 50 was triggered a year ago,” he says. “It expires in 52 weeks and a few days, and I don’t think there is any realistic prospect of it being revoked. Therefore we will be leaving the EU in March 2019.”

So there it is. The man in charge of the Brexit policy of a pro-European opposition party that could soon be in government, that campaigned for Remain, and two-thirds of whose supporters backed staying in the European Union on referendum day, holds out no hope of reversing the decision. He takes no pleasure in saying so. “I campaigned to stay in the EU. I voted to stay in the EU and I was very disappointed by the outcome. And if there was another vote I would vote to remain in.”

But he doesn’t think there will be a second referendum, nor does he seem to believe there should be one. Starmer may regret deeply what will come to pass on 29 March next year, but he feels equally strongly that what must be must be. “Having asked the electorate for a view by way of the referendum, we have to respect the result,” he says. “If you find yourself in a position you would rather was not there, you have to make it work. We have to do that for the current generation and for future generations.”

Many Labour MPs and supporters will be dismayed to hear him talk this way and he knows that. “Is it difficult? Of course it’s difficult,” he says. “Almost everybody in the Labour party has a view on Brexit. But almost no (two) people have the same view. They all give me their opinions all of the time in texts, in emails, in one-to- one conversations, in groups.”

From the top to bottom of Labour there are differences of view. Jeremy Corbyn may inspire young people but he is a Eurosceptic at odds on Brexit with much of the youthful mass membership that is his powerbase. The left is split within its own ranks. Most of the unions that fund Labour are in favour of staying in the single market, but Corbyn seems implacably opposed. Wherever Starmer treads, there are competing demands he has to try to satisfy. “You have got the basic mathematics that show that, broadly speaking, two-thirds of our [Labour] voters voted to remain and one-third voted to leave. You have then got the flipside of that when you get to the constituency representation, which is the other way round.

“Two-thirds of our MPs are in Leave seats and one-third are in Remain seats, and MPs quite rightly feel strongly that they should be trying to put across the views of those they have been elected by. That inevitably means there are different views.”

It is not just the Leavers versus the Remainers. “There is also the matter of how close people think we should be to the EU [after Brexit]. There are different views in the different groupings in the Labour party.”

The only thing he can do, he says, is try to keep the party together by managing the multiplicity of opinions. His objective is to prevent Labour splitting on Brexit as the Tories have for decades over Europe. “My view has been informed by my strong belief that we really cannot allow the Labour party to divide and break up on this issue. We have got to hold the party together, and of course that means there are huge challenges.”

As if to make the point for him, on the day we speak his shadow cabinet colleague Owen Smith is sacked by Corbyn for saying Labour must back staying in the single market and a second referendum – both of which are against current party policy.

So given this constant internal tug-of-war, what kind of Brexit does Starmer himself want? And what does he believe is deliverable after March next year, and then after the transition period ends in December 2020?

If anyone can finesse an argument it is Starmer. He left a stellar career in law which saw him scale the heights of that profession to become director of public prosecutions before entering parliament in 2010. But while his legal past is an asset in many ways, it has caused some in his party to accuse him of being too much the lawyer, too little the brave politician giving a lead. He is charged by some with favouring policies that amount to no more than “constructive ambiguity” just to keep peace in his party.

Naturally, he strongly contests that view and says that, step by step, a Labour Brexit plan is taking shape that is increasingly bold and distinct. It is one, adds Starmer, that backs close post-Brexit links with the EU and so sets Labour well apart from the hard-Brexit Tories. He has already dragged Corbyn, the shadow chancellor John McDonnell, and the shadow trade secretary Barry Gardiner round to the view that Labour should back remaining in a customs union with the EU post-Brexit. It is the only way to resolve the problem of the Irish border, he says. In a speech tomorrow he will announce plans to rally cross-party support behind an amendment to the withdrawal bill which would enshrine into law a commitment to avoid any form of hard border.

But no sooner had he delivered on the customs union commitment a few weeks ago than Starmer faced a barrage of demands from the large pro-EU wing of the parliamentary Labour party to go a big step further and support keeping the UK in the single market, too. Corbyn and McDonnell will not wear that, as evidenced by the summary dismissal of Smith on Friday night. Plenty of pro-single market Labour MPs say Starmer could honour the referendum result (and take the UK out) while remaining in the single market by joining the European Free Trade Association, like Norway. He rejects that route, saying that what suits Norway would not suit the UK. “Why would any country want to borrow the model that another country has that suits their economy and their interests?” he asks.

Instead he is clear that he wants a “bespoke” UK deal with the EU. “We need no persuading of the benefits of the single market and customs union. That is why our manifesto was to maintain the benefits of both. That is not a throwaway commitment. We want the benefits of both to be hardwired into the final agreement,” he says. This sounds at first like having your cake and eating it. Starmer knows as well as anyone that full access to the single market, with all its benefits, would require the UK to agree to EU free-movement rules, opposition to which was the main reason millions of Labour voters backed Brexit.

So what is he thinking? He hints at a possible arrangement, a potential area for compromise, under which the UK could, under a Labour government, meet the EU at least halfway on free movement by giving its citizens preferential terms for coming to the UK to work, over those from non-EU states. Again, he suggests progress is possible only if the party can be held together. “What we are focused on is what is needed for our economy and our communities – in other words a principled, humane and effective approach [to free movement], not an approach – like the government’s – that simply says we should keep to a certain number.”

Over the next few months there will be plenty of flashpoints on Brexit that will expose Labour’s divisions and tensions, as well as Tory ones, and Starmer will be in the middle of them all. Some time after Easter the House of Lords is likely to back amendments calling for the UK to remain in the single market. What will Labour then do when these are voted on in the Commons? Will it whip its MPs to strike them out of the withdrawal bill, or back them?

Starmer ducks the question, saying it depends what the amendments actually say. It is a nightmare issue for a man who, many colleagues believe, would love to be able to back staying in the single market if only his leader would allow it.

But at the same time he rules nothing out. Starmer seems unfazed by the near impossibility of his task. But he does bear the scars of trying to honour the referendum result while forging a policy that would keep the UK as close as possible to the EU after it leaves.

The most exhausting part of the whole process, he says, was over the decision Labour made last year to back triggering article 50 – the process that put the country on course for the Brexit he never wanted.

Starmer was evidently tortured by that decision and it now seems to haunt him with a year to go until exit day. “We are a pro-EU party, we campaigned to stay in the EU. It was a difficult and draining time for all us of us.”