Europe is watching events unfold in London with fascination, frustration, and no small measure of helplessness.

Simply put, it is impossible for the EU to predict how it will react to the House of Commons vote because the situation is so fluid.
"We'll have to see what the UK proposes and only then can you try to figure out what the next days will bring," said a senior EU source.

European Council President Donald Tusk is expected to issue some kind of reaction, possibly as early as tonight.

There was supposed to be a debate in the European Parliament today on December's EU summit, but that has been cancelled because President Tusk has had to travel to Gdansk following the murder of the city’s mayor, Paweł Adamowicz.

The European Parliament will have a full debate on Brexit tomorrow morning during its plenary session in Strasbourg.

Michel Barnier, the EU's chief negotiator will speak, as will the leaders of the main political groups, so there will be a flavour then of EU reaction.

But the really critical response will be from the European Council.

The Commission has done its work on the Withdrawal Treaty, so if Theresa May is to make a plea for more help following tonight's vote it will be to the 27 leaders.

But the options are very limited.

"The EU’s room for manoeuvre is close to zero," said a senior official in the European Parliament. 

"The [Tusk Juncker] letter yesterday demonstrated that beautifully. We can re-emphasise this, re-underline that, but the room for manoeuvre is limited." 

Europe's response will be calibrated on the scale of Mrs May's defeat.

"Will she have a serious defeat, or what she will consider a manageable defeat?" says another senior EU official.

"I think if this is a crushing defeat quite honestly I can't see how she will survive, because it will trigger all kinds of events, starting with resignations in the cabinet, then a rather chaotic 48 hours and potentially some kind of gridlock."
A smaller, "manageable" defeat would prompt a period of reflection, suggests the official, during which Mrs May might have to reach across to Labour to see if the deal can be salvaged with their support, perhaps with the promise of a permanent customs union with the EU.

But that in turn would prompt the collapse in support of her own back benches.  Indeed, the various tribes in the Commons hate the deal for different and contradictory reasons, so Brussels will need to keep its options open.

The EU (and especially Dublin) will be nervously eyeing any amendments - such as the Swire and Murrison amendments - which, in exchange for supporting the Withdrawal Agreement, would try to clamp an expiry date on to the backstop, or require the House of Commons to give its permission for the backstop to take effect.

The EU could probably turn a blind eye to some "innocuous" parliamentary mechanisms that would be entirely for domestic political reasons, but anything which ran counter to the text of the Withdrawal Agreement concerned would be a major difficulty.

"That is far more problematic," said a senior source, "because it would indicate they are not prepared to fully implement their international obligations on the backstop, under the Protocol and Withdrawal Agreement. That’s a real problem. That’s going clearly against what is agreed."

That would almost certainly scupper the deal, because the EU would refuse to sign the treaty if there was an ex post facto threat from the Commons to undermine it.
"The backstop cannot be conditional on a vote in the House of Commons," says another EU official. 

"It has been said many, many times. It’s an all-weather backstop.  You can’t have conditionality or an event that is outside the control of the parties, there’s no agreement."

Were Theresa May to seek an extension of Article 50 that would certainly be considered. 

Officials from both the European Commission and the European Council have already been looking at the legal ramifications of such a request.
The main problem is that the European Parliament elections are on 25 May. The new parliament will then be fully constituted on 2 July, so on paper 1 July would be the limit to any extension.

If the UK needs an extension beyond that, there are legal problems. If the UK remains a member state through the summer and into the autumn, it will have to have MEPs.

That would mean the bizarre situation of a UK election campaign for the European Parliament in the autumn, after the other countries have voted, potentially colliding with a second referendum or a general election.

It would also mean that the redistribution of Strasbourg and Brussels seats - of which Ireland has already been a beneficiary-  would have to be reversed, and it would distort the composition and political balance of the parliament.

Officials would also ruefully have to contemplate the regular European Elections in May being spiked by the Brexit chaos.

"The last thing we need is an importation of the Brexit problem into our own elections," says a European Parliament official. 

"If the Brits had not yet left, three years after they had voted to leave the EU, you can imagine the campaign of Marine Le Pen and others, saying Brussels has confiscated the will of people as it were, and stopping the Brits from leaving."