In 1985, a young MP – only two years in the job – stood up in the House of Commons to oppose the government’s plans to withdraw from Unesco, the UN agency created after the second world war to encourage global development and collaboration in the fields of education, culture and science. Responding to the Thatcher government citing Unesco’s bureaucratic and budgetary problems, he said: “The basis of this debate is not the finances or the administrative arrangements of Unesco, but the power of the far right in the United States.”

That was Jeremy Corbyn, the only Labour MP who took part in that debate who is still standing in the House of Commons today, and not for the first or last time standing on the right side of history. But now, history is repeating itself, with the news that Penny Mordaunt, the international development secretary, plans to follow the recent lead of Donald Trump and Israel’s Benjamin Netanyahu by once again withdrawing the UK from Unesco, 21 years after Robin Cook restored our country’s membership.

As in 1985, she is blaming the agency’s budgetary problems, but – again, just like then – it is nothing but a smokescreen for the Tories’ shameful obsession with pleasing this US president. When the Trump administration announced its withdrawal from Unesco jointly with Netanyahu’s last year, it was explicit about the supposed “anti-Israel bias” that had driven its decision, by which it meant Unesco’s temerity in accepting the overwhelming vote of its members in 2011 to admit the state of Palestine as a full member. For Mordaunt to blame concerns over Unesco’s finances instead is not only to take the public for fools but also a surprisingly cowardly move for a minister who is usually willing to speak her mind.

In doing so, however, she has contradicted herself. Only nine months ago, she praised Unesco’s new director-general, Audrey Azoulay, as being “very focused on the reform agenda”, and reassured her that “we will support her” in the “tough job” ahead. What has changed in nine months? And what has changed since, only last month, Matthew Lodge, the UK’s ambassador to Unesco reassured his colleagues of the UK government’s view that “although challenges remain, Unesco’s finances are on a sounder footing. The political disputes that have plagued this organisation for too long have been de-escalated and there is a renewed sense of optimism for the future.” Four weeks on, the same government proposes to walk away.

And let us be clear what we are walking away from: the UN agency that – among many other things – is responsible for driving up literacy rates across the globe, promoting gender equality in education, protecting press freedom, coordinating tsunami warnings, and preserving more than 1,000 of the world’s most important heritage sites, including 31 here in Britain.

But even more than that, we are walking away from our own heritage. It was British politicians such as Rab Butler who helped develop the concept of Unesco, a conference in London in 1945 that framed its constitution, a British scientist who became its first director-general, and a Labour prime minister – Clement Attlee – who came up with the words that summed up its mission. “Since wars begin in the minds of men,” he wrote in the preamble to the new agency’s constitution, “it is in the minds of men that the defences of peace must be constructed.”

That was the thinking of a generation shattered by the second world war, desperate to create the global institutions that would not just prevent any repeat but for ever stop the nationalist forces of hatred, division and prejudice from rising again. Unesco can rightly be claimed as one of Britain’s greatest contributions to that global architecture of peace, and for Penny Mordaunt to be willing to destroy that legacy by withdrawing Britain’s membership is nothing but historical and cultural vandalism.

If her financial excuses were the true reason, we might just bemoan the penny-pinching and short-sightedness. But, as Jeremy Corbyn argued 33 years ago, and as the UK’s ambassador to Unesco confirmed last November, those excuses hold no water. Britain makes a net gain from our membership of Unesco: we contributed £11m to the agency this year, versus £100m value added to our economy from its designation of our heritage sites.

No, the truth is more profound and more depressing. Just as in 1985, we have a Tory government that is slavishly following America down a path to isolationism, and an ever-increasing detachment from the institutions that should govern the globe.

And if it will not listen to the Labour party, it should perhaps listen to the last truly internationalist Tory prime minister, who spoke in that same debate as Jeremy Corbyn in 1985, and who would be turning in his grave at the shame of his successors today. In the Unesco decision, Ted Heath saw: “the growth of a nasty, narrow-minded nationalism, which believes that we can survive without the rest of the world and, in its more arrogant form, that we can just tell the rest of the world what it should do. We think that we can tell the rest of the world, “Do what we say, or else we go”, implying that our loss is so great that they are bound to do what we say.

“In the modern world,” he concluded, “nothing could be further from the truth.” How right he was. And we can only ask why the Tory party still will not listen.

• Emily Thornberry is the shadow foreign secretary