ope again. That was the prime minister's message to the country this week. A national social care system, greater public control of water and energy, help for first-time buyers and a crackdown on rogue landlords and empty homes. With clarity and conviction, Andy Burnham set out a bold plan to put Britain on a new path, steeped in Labour values.

It felt like a conference speech that would be remembered for years to come, marking a decisive break with a broken political model that has let British people down for decades. And perhaps most encouraging of all was the prime minister's promise to rethink Britain's relationship with the EU.

Burnham's words were simple but stark: "Brexit has done more harm than good." He is right – and the reality of the vote's impact on the UK is sobering. Research commissioned by City Hall in 2024 showed that London's economy was £30bn smaller than it would have been without Brexit and that there were nearly 2m fewer jobs overall in the UK as a result of us leaving. Earlier this year, we set out new analysis showing that our country has grown more slowly than our comparable European counterparts since 2016. If that trend continues, our economy could be £250bn smaller than similar EU countries' by 2030.

Ten years after the referendum, the sunlit uplands promised by the Brexiters have proven to be a total fiction. Instead, Britain finds itself battling an economic and geopolitical storm.

Earlier this year, I warned that Donald Trump's bombing of Iran could have catastrophic consequences for Londoners, and it gives me no pleasure to be proved right. The folly of this reckless war of choice is now clear for all to see. The conflict is sending shock waves through the global economy, hiking up mortgage costs and pushing up fuel prices. The price of diesel has hit an all-time high at nearly £2 per litre, while energy bills are forecast to rise 25% this winter, piling unbearable pressure on many households across the UK already grappling with a cost-of-living crisis.

At the same time, Trump's fondness for unilateralism has shaken the foundations of the transatlantic alliance on which our national security depends. As Russia's president, Vladimir Putin, persists with his illegal and unconscionable full-scale invasion of Ukraine, Trump's unpredictable actions and unsettling ambivalence towards European security can no longer be ignored.

In this geopolitical storm, Europe offers us not just a safe harbour, but a huge opportunity for growth. The EU is our biggest trading partner and our most reliable ally. That's why, at the UK-EU summit in November, I will call on the government to lay the groundwork for Britain's swift return to both the customs union and the single market – a move that should happen within this parliament.

Rejoining the customs union and the single market would deliver an immediate boost to economic growth, improving living standards for communities across the country and helping to grant Londoners some respite from the most acute cost of living pressures. In the long term, however, we must raise our ambitions higher still. Our party must immediately start making the case for rejoining the EU, so we can fight and win a general election with EU membership effectively on the ballot paper in 2029.

The brutal truth is that leaving the EU was the biggest act of economic and geopolitical self-harm in our country's history. Rejoining is the only viable path to long-term prosperity and security. Anything less will fail to undo the severe damage that's been done.

For far too long, the Labour party's debate about Europe has been constrained by "red lines" that make rebuilding our relationship with the EU impossible. But Burnham has now recognised that these lines were drawn up for a different world.

In Canada, Mark Carney has set out his plan to partner much more closely with the EU on defence, critical minerals and energy security, with the country now on the path to becoming the bloc's first associate member. In this multipolar world, middle powers such as Canada, the UK and our European neighbours must come together, both to safeguard our own security and economies and to stand up for free trade and our liberal democratic values.

Deeper integration can – and will – unlock a new age of growth and productivity, but it will also provide us with a louder voice on the world stage. From bolstering our military defences to safely developing AI and combating the climate crisis, if we are to make ourselves heard, we must speak as one. And in a world whose superpowers appear ever more comfortable with throwing their weight around, the EU is the only organisation with the economic and geopolitical heft to hold its own.

For me, the most memorable line in Burnham's speech came a couple of minutes in: "Hope comes from honesty." They were the words of a leader unafraid to change the terms of Britain's dysfunctional political debate by taking on the seemingly intractable problems that too many would be inclined to avoid.

Today, Brexit is foremost among those problems. If we are to bring back hope, we must be honest about the disastrous costs of our decision to leave the EU. And we must be honest about the fact that reversing it is the best chance we have to get Britain back on track – and get the country firing on all cylinders again.

• Sadiq Khan is mayor of London