Victory is fleeting. Theresa May’s bombshell that it was the last Labour government not her, who oversaw the decision to destroy the landing cards that would have provided proof of residence to Generation Windrush successfully floored Jeremy Corbyn in the chamber but is rapidly coming unstuck under further scrutiny.

Downing Street has admitted that the last Labour government had nothing to do with it: the decision was signed off by officials at the Border Agency, not ministers of any hue. Their destruction was, in any case, implemented under the Conservative government of Theresa May.

As George explains in greater detail here, May’s attack line didn’t really make sense anyway. The decision to destroy the cards in 2009 was an act of cultural and historical vandalism: they should be in a museum or an archive, at least in a digital form. (Bloomberg’s Rob Hutton makes the excellent point that the value of the cards would only have increased to future generations, who may have wanted to buy their great-grandparents’ card at some future date.) But it gained a new political dimension in October 2010, because May already wanted to increase the bureaucratic burden upon immigrants. The destruction went from an act of vandalism to one of cruelty on the part of the Border Agency.

Still, Corbyn failed to explain that so she got away with it in the chamber. But could she now be in jeopardy after lying to the House?

But May chose her words carefully: she talked of the decision being made “under” Labour rather than “by” Labour, which is technically true, even though she conveyed an impression that things were otherwise. So she lives to fight another day.

But there is a broader problem here that should trouble Conservatives as much as the opposition: of a gradual eroding of the norms about how the executive behaves with reference to parliament. Choosing your words so you do not lie but do not convey the truth is not a resigning offence: but it isn’t good for public discourse or constitutional safeguards either.