BRUSSELS (Reuters) - The European Commission opposes extending British membership of the European Union to June 30 as Prime Minister Theresa May proposed on Wednesday, according to a document seen by Reuters.

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In a Wednesday note, Commission officials said EU leaders meeting May at a summit on Thursday faced a “binary” choice of a short Brexit delay from March 29 to May 23, or a long lag to at least the end of this year, with Britain obliged to hold European Parliament elections in May.

“Any extension offered to the United Kingdom should either last until 23 May 2019 or should be significantly longer and require European elections,” the document said. “This is the only way of protecting the functioning of the EU institutions and their ability to take decisions.”

EU states due to receive additional legislative seats after Brexit would need to know by mid- to late-April if they would be denied those seats because Britain was staying.

The note also said in any extended membership, Britain should, “in a spirit of loyal cooperation”, commit to “constructive abstention” on key issues, such as the EU’s long-term budget and filling top EU posts after the May election.

Commission President Jean-Claude Juncker warned May in a call on Wednesday against requesting a delay beyond May 23, unless the United Kingdom takes part in the European vote, an EU spokeswoman said.

Most EU states vote on Sundays and will vote on May 26 but Britain votes on Thursdays, and so would vote on May 23.

LEGAL UNCERTAINTY

“President Juncker has formally warned the PM against including a date for the extension that is after the EP elections,” a Commission spokeswoman said.

“That’s why he repeated to her his advice ... that the withdrawal has to be complete before 23 May - otherwise we face institutional difficulties and legal uncertainty. European Parliament elections have to be held (in the UK) if the extension date is after 23 May,” she said.

The EU is wary of undermining the legitimacy of the new European Parliament - which convenes on July 2 - should Britain eschew the vote in May but remain a member of the bloc for longer, denying its citizens the right for parliamentary representation picked in a direct vote.

Any legal doubts over the new parliament would risk poisoning its next decisions, including on the appointment of a new European Commission later this year or on the EU’s budget.

“Every decision would be open to legal challenge,” the Commission said in its note.

The Commission, which may hold a stricter view on the matter than some of the 27 EU national leaders who would make any final decision, also said a long Brexit delay could last until the end of this year or longer and could be cut short if a solution were found earlier.

“Under all circumstances... the European Council should envisage a single extension, rather than a series of extensions, which would keep the European Union in limbo for an extended period,” it said.

It insisted any leniency could be granted to Britain only on condition that it would not seek to use the extra time to renegotiate the stalled divorce deal.