A Brexit delay of longer than two months could be illegal unless the UK elects new MEPs, lawyers are warning, appearing to torpedo Theresa May’s strategy.

Article 50 cannot be extended beyond the end of May unless the UK takes part in fresh European parliament elections, according to a legal opinion issued by the German Bundestag.

The conclusion came amid fresh evidence that the exit talks remain in trouble, with a lack of progress that might persuade MPs to approve the prime minister’s deal.

The controversy over European elections is crucial because Ms May told MPs a delay to June would be possible without a U-turn on withdrawing MEPs – if parliament forces her to abandon the 29 March deadline.

A proposal to keep seats in the parliament would trigger uproar among Conservatives who are already angry over the Commons being granted a veto over a no-deal Brexit.

However, the EU would almost certainly reject a delay of only two months as pointless, given it has not been possible to agree an acceptable UK divorce deal in two years of trying.

The blow came as Michel Barnier, the EU’s Brexit negotiator, is believed to have warned there has been no progress in the negotiations designed to secure changes to rescue the agreement.

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A regular meeting of EU ambassadors was told there was still stalemate over the UK’s search for an exit mechanism from the Irish border backstop, which the EU has rejected, it was reported.

On Tuesday, Ms May – while insisting she remained opposed to delaying Brexit – told MPs: “An extension beyond the end of June would mean the UK taking part in the European Parliament elections.”

However, the legal opinion warned of “a violation of the active and passive voting rights of British nationals” if there was a delay beyond 23-26 May, when the elections are held.

“British citizens living in the UK would be denied a fundamental right conferred on them by citizenship of the European Union,” the German paper Die Welt quoted the advice as saying.

They could take action in UK courts “in defence of their rights to vote and stand for election … in accordance with applicable national rules”.

And EU citizens in the UK would also be prevented from “exercising their right to vote and stand in elections to the European parliament in their state of residence”, laying Britain open to possible action by the European Commission.

The European Court of Justice could issue a preliminary ruling on “whether or not Britain’s non-participation in the European elections is legally permissible”.

It is increasingly plausible that the EU will insist on a much longer extension – possibly 21 months, to allow a trade deal to be agreed while the UK is a member, avoiding the backstop controversy.