BRUSSELS (AP) — British Prime Minister Theresa May, who will be traveling to European Union headquarters to seek an elusive breakthrough in Brexit negotiations, was told Tuesday on the eve of the talks that her most important demand is not even up for discussion.

EU Commission spokesman Margaritis Schinas said "the EU27 will not reopen the withdrawal agreement," a condition that many British lawmakers have insisted upon before they will back a deal to have Britain leave the bloc on March 29.

With time getting tight and British legislators growing increasingly uneasy over the government's Brexit plans, May is still looking to change the deal's legal phrasing to make sure that the conditions she agreed to on the border with EU member Ireland would bind the UK only temporarily.

The EU refuses to budge and says the 585-page legal Brexit agreement is a take-it-or-leave-it document. It does want to discuss other ways to find a compromise, but, so far, London has been insistent on changing the legal text.

Schinas said talks this week seek "to see whether a way through can be found that would gain the broadest possible support in the U.K. parliament and respect the guidelines agreed" by the EU member states.

Under the current Brexit deal negotiated between May and the EU, there would be a long transition to make sure businesses and trade suffer as little as possible. But if no Brexit deal is agreed to by the British parliament, that risks a chaotic departure that could be costly to both sides — both to businesses and ordinary people.

As the time shrinks between a Brexit deal and the Brexit departure date, the more difficult it becomes for businesses and authorities to adapt to the fundamental changes that Britain's withdrawal from the EU would entail.

The British Parliament voted down May's Brexit deal last month, largely because of concerns about a provision for the border between the U.K.'s Northern Ireland and Ireland. The mechanism, known as the backstop, is a safeguard that would keep the U.K. in a customs union with the EU to remove the need for checks along the Irish border until a permanent new trading relationship is in place.

Both sides could also decide to postpone the departure date by several months, but if it is extended too far, it would force Britain to take part in the May 23-26 EU-wide election for the European Parliament.

EU Commission President Jean-Claude Juncker told the German daily Stuttgarter Zeitung on Tuesday that such a scenario was "difficult to imagine," saying it would be a "belated joke of history."

Juncker said it's up to Britain to decide whether it wants to request a delay to the Brexit date, but that it should happen before the newly elected European Parliament gathers in early July.

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Corrects that first EU plenary is in July, not June.

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Frank Jordans contributed from Berlin