Prerogative power just took a big hit.

Right now, everyone is looking at the Article 50 issue, quite rightly.  But before addressing that, it’s worth pointing out that the executive in Britain uses prerogative powers widely.  A minority of geeky MPs have long complained about it, saying it undermines Parliamentary sovereignty.  They got nowhere: governments were not interested naturally, and neither were voters.

But now it sits centre stage.  From here on, today’s ruling will be potent for those seeking to roll back prerogative power across Whitehall.  Unless the Supreme Court overturns today’s verdict.

Meantime, what does Parliament do with its enhanced role in respect of Article 50?

Passionate Bexiteers have been quick out of the traps, warning that MPs must not seek to overturn the expressed will of the electorate.

In terms on invoking Article 50, they almost certainly won’t.  The withdrawal process will get underway.

What’s changed is this: the Government had sought to get the process under way without being encumbered by any negotiating conditions, and by keeping its approach to those negotiations largely under wraps.  If today’s judgement stands, that isn’t going to happen.

Although the majority of MPs are for Remain, Parliament won’t vote to prevent the triggering of Article 50.

For the Government, Mrs May cannot go back on ‘Brexit means Brexit’.  Conservative MPs will have to back her on that. The eurosceptics would anyway.  The Europhiles will have to, because the alternative would be revolt in their local associations and almost inevitable deselection.

For Labour, voting to block the Article 50 trigger would result in the permanent loss of millions of votes, as Labour Brexit supporters would be totally confirmed in their sense of betrayal.

There is force in the simple observation that ‘the people have spoken’.

What changes as a result of today’s judgement is that now conditions may be attached to the authority to trigger Article 50.

The pro-Remain majority in Parliament may be able to coalesce round some conditions that would impose far more challenging parameters around the negotiation process than Ministers ever expected.

We can now anticipate stacks of amendments designed to ensure certain outcomes.  The Government’s worst nightmare is that Parliament will also secure an amendment which ensures that the ultimate result of the negotiations has to come back to Parliament  for a definitive approve/disapprove vote.

If Parliament succeeds in turning the Article 50 trigger mechanism into a ‘Christmas tree resolution’ festooned with all manner of decorations, the exit negotiations will become a lot more difficult.  That will be the intention of those supporting the amendments.  Their public line will be, ‘of course we are not frustrating the expressed will of the people, we are simply trying to ensure the best outcome’.

But of course that may immediately become harder to achieve.

A series of benchmarks will be set down by which to judge the outcome of the negotiations.  The two-year negotiation process will be an exercise in navigating through the whole collection of EU member state interests, with a large part of the UK’s negotiating position on show from the outset.

The EU could play a canny game.  It could ensure that the conditions attached by Parliament are not met, on the assumption that Parliament would then have to reject the deal, and the UK’s departure would be blocked.

Then the question is, would voters accept that, or cry betrayal and double-down on their bid to get the UK out?  The outcome might depend on what the economy has been like over the next 48 months.

Mrs May is unlikely to come unstuck over actually getting approval for Article 50.  So she won’t have to seek a new mandate for that.

But she could come unstuck on securing approval for the eventual exit terms after the two-year negotiation.  Then, in 2019, we would have Act 2 of the Brexit tragicomedy.