A spokesperson for the European Commission suggested that the Tory MP was essentially irrelevant and not involved in negotiations.
“This gentleman is not our interlocutor and I would say then that the principle of sincere cooperation does apply, as prime minister May herself makes clear in her letter,” the spokesperson told reporters in Brussels.
“I would also say that this is a hypothetical question because it supposes, or presupposed an extension, which is yet to be seen by our leaders.”
Guy Verhofstadt, the European parliament’s Brexit coordinator, also piled in. Seizing on Mr Mogg’s comments, he said: “For those in the EU who may be tempted to further extend the Brexit saga, I can only say, be careful what you wish for.”
Mr Rees-Mogg, a leading figure in the eurosceptic ERG group of Tory MPs, had said on Friday morning that “if a long extension leaves us stuck in the EU we should be as difficult as possible”.
He suggested: “We could veto any increase in the budget, obstruct the putative EU army and block Mr Macron’s integrationist schemes”.
Theresa May requested a further delay to the Brexit date, which will be considered by the 27 remaining presidents and prime ministers at a special meeting in the Belgian capital on Wednesday. It is understood that EU officials are considering offering Britain a one-year extension until the end of March 2020, with the ability to cut it short if the UK ratifies the withdrawal agreement by then.
The prime minister has requested only a short extension until the end of June, but is thought ot be unlikely to get it. A longer extension as favoured by the European Council would see the UK participate in European parliament elections if it was still in the EU by the 22 May.
After last month's summit a senior EU official said the bloc was well aware of the risk posed by Brexiteers who might be minded to deliberately cause chaos in Brussels if the UK remained.
“On the one hand, the UK has been behaving in accordance with the obligation of sincere cooperation until now. And at the same time… we do also read the British press, and we have seen some comments that maybe we should take this opportunity to break the union from the inside," the senior official said.
“I don’t think that will be possible, and I think any British government will understand that that’s not a promising way forward. We luckily have qualified majority voting on most issues these days and if we don’t, I’m sure we’d find ways of dealing with it if these things were to happen.”
Various senior EU figures, including Emmanuel Macron, have said any further extension of Article 50 would require a justification, which could be time to hold a general election, second referendum, or a political process in the UK to reach a majority.
The prime minister's team is currently locked in talks with Labour in an attempt to bring about a cross party consensus in Westminster.
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