The European Union has made no secret that it thinks Prime Minister Theresa May should delay Brexit day. The question is, for how long.

May on Tuesday promised the British Parliament a vote on a “short” postponement if it doesn’t approve her deal by March 12. EU officials are relieved it lessens the chances of a crash-out Brexit at the end of next month but say there must be a clear plan for what’s going to happen during the extra period. And because all 27 remaining leaders have to give their consent, they get to dictate terms.

EU President Donald Tusk said an extension would be ” rational” but the problem is that there isn’t a united position among the 27 EU countries on how to play the negotiation and how long an extension to offer. Most governments and the European Commission oppose the prospect of a rolling three-month “serial extension,” which they fear would merely prolong uncertainty and lead to episodic crises and which May’s decision raises the prospect of.

That’s why some governments and senior officials in Brussels are now privately talking up the need for a long extension if May doesn’t get the deal approved by Parliament before the scheduled March 29 day of departure.

Still, if the U.K. insists on a short delay, the EU may not be able to prevent it.

Some EU officials have floated 21 months — which would replace the already-agreed transition period throughout that time, and last until the end of 2020. Other governments favor a postponement of one year, or until the end of 2019.

Tactical Play

This is in part a negotiating tactic — a bid to persuade pro-Brexit lawmakers in the U.K. Parliament to back the deal May struck last year out of fear that a long extension would end up killing Brexit altogether.

Some governments still pin their hopes on the U.K. changing its mind to stay in the EU, or at least softening its red lines to remain much closer to the bloc. They want a long extension to give the U.K. time to change its mind, according to European diplomats. Opposition Labour Leader Jeremy Corbyn’s move on Monday to back another vote boosts this argument.

EU leaders will probably have to thrash out a common position going into their summit next month if May’s deal isn’t approved by then. EU officials say it’s very unlikely that a request for some kind of delay, even though it needs the unanimous support of all the bloc’s leaders, would be refused.

But there’s another complicating factor: European elections to be held at the end of May. Any extension beyond three months — because the new European Parliament convenes on July 2 -– would probably mean that the U.K. would have to take part, which officials in the U.K. and the EU fear could add further turmoil to an already fragile political situation.

If May gets her deal through before the end of March the outlook changes. The EU still thinks a postponement is inevitable, and expects the U.K. to ask for three months to get outstanding legislation through Parliament.