BRUSSELS (Reuters) - Any long Brexit delay would pile on economic and political costs for the European Union, the bloc’s chief negotiator Michel Barnier said on Tuesday, stressing that these would need to be weighed up carefully against potential benefits.

European Union's chief Brexit negotiator Michel Barnier holds a news conference after a General Affairs Council on Article 50 in Brussels, Belgium March 19, 2019. REUTERS/Yves Herman

Barnier spoke ahead of an EU leaders’ summit on Thursday and Friday where they could decide on postponing Brexit beyond the current date of March 29, should British Prime Minister Theresa May request that.

“It is our duty to ask whether this extension would be useful. It would extend uncertainty and uncertainty costs. We cannot prolong uncertainty without having a good reason for it,” Barnier told a news conference 10 days before Brexit is due to happen.

“EU leaders will need a concrete plan from the UK in order to be able to make an informed decision,” Barnier said.

He said the other 27 national leaders would ask May whether delaying Brexit would increase the chances that her fractious parliament would ratify their stalled Brexit deal.

Barnier said the EU would assess what is best for itself.

“Extending uncertainty without a clear plan would add to the economic costs to our businesses but could also incur a political cost for the EU,” he said.

May is expected to formally ask the EU to delay Brexit by at least three months.

Barnier warned, however, that she could not seek a short and a long extension at the same time.

“A longer extension needs to be linked to something new, there needs to be a new event, a new political process,” he said.

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The EU has pondered a Brexit delay as long as until the end of 2020 - when the bloc’s current long-term budget ends - if London were to change May’s approach and seek much closer ties with the bloc after leaving, hold a second referendum or elections.

Diplomats and officials in the bloc said, depending on what exactly May asks for, EU leaders could hold off on making a final call this week, helping May put pressure on her MPs to back her divorce deal at the third attempt before March 29.

Other options include a short postponement until the end of June, though the EU does not believe that would be enough to break the Brexit impasse in the United Kingdom and fears London would then ask for further delay.

The bloc insists Britain would have to take part in European Parliament elections due on May 24-26 if it were still to be a member of the bloc beyond June 2, when the new chamber convenes.

But it worries about opening the way to legal challenges to the parliament’s legitimacy should Britain eschew the vote but remain inside the EU longer. The EU must agree unanimously to any London request for a Brexit lag.

The EU on Tuesday finalised legislation to lean on in case of the most-damaging no-deal Brexit and mitigate the worst disruptions in fisheries and transport between the bloc and the UK, as well as prolong student exchange programmes.

Barnier reiterated the EU’s readiness to upgrade the political declaration on future UK ties - which forms part of the troubled divorce package - should that help build a majority for the deal in the divided House of Commons.

He said the EU would not renegotiate the main legal withdrawal treaty.

“Voting against no-deal does not prevent it from happening,” he said of last week’s votes in the House of Commons in which a majority of MPs rejected leaving the EU without an agreement in place to manage the process.

“Everyone should now finalise all preparations for no-deal scenario. On the EU side, we are prepared.”