I’ve thought hard about my role in the Labour leadership election. I considered staying neutral because, as mayor, I need to work with everyone to get the best deal for London. But I’ve been asked how I’ll vote by many of the Labour members and supporters who helped me throughout my campaign, and they deserve an answer.

Ditch Jeremy Corbyn before it’s too late, Sadiq Khan tells Labour Read more

I played no part in the Labour turmoil earlier this summer. I’ve had the honour of being elected as a Labour councillor, MP and mayor, thanks to the hard work of Labour members, and I believe that the will of our membership should be respected. I value loyalty, and believe that internal disagreements shouldn’t be voiced in the media – because divided parties lose elections. But there is now an open and democratic contest for the leadership, and Labour members deserve to know how I intend to vote.

Jeremy Corbyn is a principled Labour man. I’ve been disturbed by the nasty abuse that has been hurled in all directions over the last 12 months. There should be no place for this in our party. I nominated Jeremy for leader last year – but did not vote for him – and I do not regret nominating him because party members deserved that choice. His campaign last summer was a breath of fresh air and offered hope to many.

I have little time for those who say that Jeremy is only leader because of “entryism”. It is undoubtedly a good thing that our party membership is growing. Vibrant political parties are vital to the health of our democracy. Our new members, like all of us, are desperate for a Labour government to make Britain fairer.

And that is why I have decided to vote for Owen Smith – because Labour party members, and the British people, need Labour to win the next general election.

By every available measure, if Jeremy remains as leader, Labour is extremely unlikely to win the next general election. The hopes of the members who have joined our party would be dashed again. Jeremy has already proved that he is unable to organise an effective team, and has failed to win the trust and respect of the British people. We need to be honest and recognise that means it will be more likely that the NHS will come under even greater attack from the Tories. More likely that we will see even deeper Tory cuts. More likely that our industrial heartlands will continue to decline, and that zero-hours contracts and job insecurity will become the norm. Because the Tories will stay in power, and Labour will stay in opposition.

Jeremy’s personal ratings are the worst of any opposition leader on record – and the Labour party is suffering badly as a result. He has lost the confidence of more than 80% of Labour’s MPs in parliament – and I am afraid we simply cannot afford to go on like this.

This failure was most starkly demonstrated in a heartbreaking way throughout the EU referendum. Like most Labour activists, I campaigned hard for Britain to stay in the EU. Campaigners told me that Jeremy was failing to persuade Labour supporters outside London, so I went to campaign in Manchester, Leeds and Bradford. I was devastated by the result, and have spent every minute of the last two months trying to salvage the best possible outcome for London and our country – and reassuring EU citizens already living in Britain that they will remain welcome.

Throughout the campaign and aftermath, Jeremy failed to show the leadership we desperately needed. His position on EU membership was never clear – and voters didn’t believe him. A third of Labour voters said they didn’t know where the party stood on the referendum just a week before polling day. And you can’t just blame a “hostile media” and let Jeremy and his team off the hook. I know from my own election – up against a nasty and divisive Tory campaign – that, if we are strong and clear enough in our convictions, the message will get through to the public. That’s a test that Jeremy totally failed in the EU referendum. Why would things be different in a general election?

To make matters worse, the next day I was astonished to see Jeremy on television calling for the government to immediately invoke article 50 and take Britain out of the EU. This would be devastating. It would cause economic chaos, a huge increase in unemployment, and would really hurt the most vulnerable people – as well as cause EU citizens in Britain terrible uncertainty over their future.

I served with Owen Smith in the shadow cabinet and he has the strongest Labour values. We were both politicised in the 1980s. Throughout that decade an ineffective and split Labour party allowed a Tory government to do untold damage to our country. On the big issues Owen and I have been on the same side of the argument, including opposing the Iraq war. Owen led and – more importantly – won our fight against the Tories’ unfair cuts to tax credits and disability allowances, which would have hurt the most disadvantaged people in our society. And poll after poll shows that Owen is far more popular with the public than Jeremy – and far more likely to win the next election.

Simply opposing Tory policies will never be enough to help the people we exist to support. At best, you knock just the very sharpest edges off the Tory project. Winning elections is how you really make a difference. Only then are you in control – able to shape the agenda and implement Labour policies to create a fairer and more equal society. If we give up on winning, we give up on the very people who need us the most.

On 24 September, no matter who wins the leadership contest, the Labour party must ultimately unite again, oppose this new Tory government and provide a vision to the British public of how a Labour government would improve their lives. Talk of a split or a new party is deeply irresponsible and would make it easier for the Tories to win again.

We can’t afford to spend another moment fighting each other. We need to put all our efforts into fighting for a Labour government at the next election, and I believe that Owen Smith is the best person to lead that fight.