“Deluded”. “In another galaxy”. “The more I hear, the more sceptical I become.” These are just some of the ways the European commission president, Jean-Claude Junker, reportedly described Theresa May’s approach to Brexit following his meeting with her last week. He obviously hadn’t got the memo about strong and stable leadership.

Whatever provoked details of this meeting to be leaked, it is a remarkable and deeply worrying account, underlined by Angela Merkel’s warning a few days later of what she called May’s “illusions”.

Britain is about to embark on the most important set of negotiations it has had for decades. They will be complicated and difficult. To get a good deal, Britain needs all the goodwill possible, and to show subtlety and flexibility. At times like this, nuance and leaving options on the table are benefits, not drawbacks.

Make no mistake, May will take an election victory as a green light to continue towards a reckless Tory Brexit

Yet rather than building alliances among our EU partners and negotiators, the prime minister’s rigid and complacent approach has isolated and marginalised us. She has already distanced Britain from the single market and the customs union and shown an ideological reluctance to remaining a part of common European agencies – regardless of their benefits to Britain or Europe.

This is entirely unnecessary and deeply unhelpful to getting the Brexit deal Britain needs to protect jobs and the economy, and to build a strong future relationship with the EU. There is also a real risk that May’s approach is increasing the chances of Britain crashing out of the EU with no deal being reached, which would be the worst possible outcome.

As David Davis has confirmed, failing to get a deal and reverting to WTO terms would mean, for example, 30-40% tariffs on meat and dairy products, 10% tariffs on cars and an end to passporting rights for financial services. It would be a disaster for British businesses, jobs and our economy. It should be ruled out as a viable option, not talked up as a “perfectly OK” alternative.

This latest leak and the damning assessment it gives of the prime minister’s approach also speaks to a wider truth about the election we are fighting, and the different approach to Brexit we so badly need.

May called this election on the entirely false prospectus of “strengthening her hand” in Brexit talks. The reality is that, with every day, her rigid and complacent approach is weakening Britain’s position. It is making us more isolated in Europe. It is increasing the risk that we leave with a deal that severs us from our nearest allies and most important trading partners.

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Perhaps the most telling line attributed to Juncker was that he left Downing Street “10 times as sceptical as I was before”. Unless we change tack quickly and adopt a more constructive approach, the weaker our negotiating hand will become.

Make no mistake, the prime minister will take an election victory as a green light to continue down this disastrous path towards a reckless Tory Brexit that will put jobs, the economy, workers’ rights and the environment at risk.

That is why I have argued that we need to reset our relationship with the EU and take a fresh approach to Brexit. Accept last year’s referendum result. Prioritise jobs and the economy. Protect workers’ rights and the environment. Safeguard the rights of EU nationals in the UK and UK citizens in the EU. Build a close and mutually beneficial relationship with the EU: not as members but as partners.

Above all, show a willingness to work with the EU and the flexibility needed to negotiate a good deal that delivers for Britain and the EU.