Among the changes brought about by yesterday’s series of amendment votes, the shift in Labour’s position has been under-remarked upon. By whipping in favour of the Cooper amendment, Jeremy Corbyn has effectively changed his Party’s policy to delaying Brexit.

That’s quite a sizeable move. While the pro-EU wing of Labour is still demanding more, in the form of cancellation or a second referendum – as they always will – we shouldn’t be in any doubt that the practical effect of the amendment would have been to obstruct Brexit.

While Cooper herself denied it, that was certainly the open intention of many of those promoting her amendment, including the rebranded Remain campaign, which funded online adverts promoting the amendment ahead of the vote.

Formal denial but practical support for such obstruction is entirely in keeping with Labour’s established policy of constructive ambiguity, in that it allows a change of policy to take place without MPs officially having to change their position. They are still, according to their statements, committed to honouring Brexit – it’s just their actions which creep closer to doing the opposite.

Despite the cover of that carefully nurtured ambiguity, it was notable that on this particular vote the Labour leadership failed to bring its whole Parliamentary Party with. Fourteen MPs rebelled to vote against the amendment while a further 11 abstained.

The reasons are not simple. Some – like Dennis Skinner, who some Corbynites are now demanding be deselected for disloyalty – are rather Brexity generally. Some are frustrated with Corbyn, and do not trust the leadership strategy to successfully stay on this tightrope forever. Some represent Leave-voting constituencies and have found what they feel to be the limit of what their constituents will put up with, in terms of ignoring the result. All evidently felt there were greater prizes in not doing what their leader wanted than in being obedient.

It is notable that unhappiness about supporting this amendment extended to the Labour frontbench. Of those 11 abstainers, no fewer than six were shadow ministers – effectively getting as close to outright rebellion as they can while (maybe) keeping their jobs.

The strategy of ambiguity has served Labour well so far, bolstered as it is with careful use and the devoted faith of many of Corbyn’s personal following. However, it is now starting to be tested much more fiercely than before. Theoretical discussions can be filled with blather, and negative votes against a Government’s policies are habitual for Opposition MPs, but as Brexit Day approaches we are moving into a period of meaningful votes, and competing proposals for what to do, rather than what not to do.

The Cooper vote proved a sharp test, and pushing ten per cent of Labour’s MPs did not vote with their Leader on it. As the deadline draws closer, and the stakes escalate, that number could easily rise, particularly if Corbyn and his colleagues misjudge things, or public pressure on MPs in Leave-voting seats intensifies.