There are only so many times you can – or should – give someone the benefit of the doubt. In asking Donald Tusk and the European council for a ludicrous, pointless extension just until June, Theresa May has surrendered any right to be seen as a prime minister acting in good faith.

The truth is that the last five months have been – by far – the worst in modern British politics. Because of May. From the moment that she pulled the vote on her withdrawal agreement (because she knew that our democratic institutions would not consent to it) she has consistently placed her own, narrow interests above those of this country. She has ignored and misled parliament. She has acted as a demagogue, giving licence to those who threaten and harass MPs. She has burned bridges with our European partners and has treated the British people with contempt.

Staying just until June solves none of the problems that her Brexit deal creates. It constructs yet another melodramatic cliff-edge, which she hopes to use as yet another threat in yet another vote on the same deal that parliament has rejected three times. It will mean electing MEPs in full knowledge (hope, even) that they will be ejected just a month later. It places no deal – with all the associated economic and human harm – back on the table.

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Why has she done this now? She did not need to – parliament is in the process of legislating for a long extension, the European council does not meet until next week. What is more, this makes a mockery of the negotiations that May is supposedly conducting with Jeremy Corbyn as I write. What is the point of these discussions if she is going to act unilaterally against the expressed wishes of Labour and of parliament?

The truth is that deep down we all – even those of us who have up till now given her the benefit of the doubt – know why. She has done this now because she cannot let go of the idea that she may yet triumph in her borderline abusive relationship with parliament and the country. Failing that, she hopes to place the blame for a long extension on everyone except herself.

For months now I have travelled the country, speaking three or four times a week at meetings about Brexit. The people who show up to town and village halls in the rain on a school night to join me are desperately afraid. I tell them not to worry, that May is a ball of wool that unravels – without exception – every time she moves. That is true of this latest outrage. She will not get what she is demanding. But the truth is that it is no longer merely May or her degenerate party that unravels any more. It is this country. Our economy, our wellbeing, our status in the world.

May is now systematically undermining the national interest. Jeremy Corbyn, our parliament, Donald Tusk and our European partners can stop her. The time has come to pull the plug.

• Andrew Adonis is a Labour peer