His outburst came after May angered some European leaders by telling her first EU summit that she expects Britain to be at the centre of European decision-making until Brexit takes place.
Brexit was a relatively low-key issue at a summit dominated by other matters, including Russia, Syria and the migration crisis.
But, in a brief speech at the end of an EU leaders’ dinner in Brussels on Thursday, May said Britain wanted to continue to play a central role in meetings and decisions until it leaves the bloc.
The speech angered fellow leaders, according to Manfred Weber, leader of the biggest group in the European Parliament.
On Friday, he said: “When somebody wants to leave a club, it is not normal that such a member wants to decide about the future of this club. That is really creating a lot of anger about the behaviour of the British government.”
The German deputy added, “If you want to leave please do so, but don’t decide for the European Union.”
“It is not about institutions in Brussels, it’s about whether the British Prime Minister in the next years is still at the table, and the majority decided to not sit anymore on the centre of the European debate, and that is not good.”
His “get tough” comments come after similar warnings from German Chancellor Angela Merkel and French President François Hollande, who were both in Brussels for the summit which concluded on Friday. They warned May that the UK faces a “rough” ride over her pledge to press ahead with a “hard Brexit.”
Hollande and European Parliament chief Martin Schulz both warned that Theresa May’s insistence on ending free movement would mean an economic cost for the UK
However, Jonathan Hill, the UK’s former European Commissioner, took issue with Weber’s objection to Britain continuing to play a central role in the EU, saying, “I think Manfred is wrong about that.
“It is important that we do continue to play an active part. Not a part that is going to frustrate things that people want to do, but to show that we want to continue to have a relationship.”
In a speech at the summit, Schulz said, “Over the coming months and years, good faith and perseverance should be the watchwords. The UK remains fully part of the EU until it withdraws. Its views and right to vote are respected as before.
“But the context of that country having a foot out of the door cannot be ignored. If Britain wants out then it should not prevent the Union from tackling the existential challenges it faces and it should show understanding that others may want to deepen their cooperation in certain areas.”
The German Socialist went on, “The process of withdrawal from the EU is already taking up much of the UK’s and the EU’s time and energy. The EU needs to find a model whereby Brexit, rather than being a permanent distraction, is instead used as the catalyst for a reform process designed to ensure that where the Union has a power to act, it also has the corresponding tools to deliver effective policy.
Meanwhile, European Council President Donald Tusk said May had confirmed that the UK will invoke article 50 before the end of March next year.
The Pole said, “There will be no negotiations until article 50 is triggered by the UK so we didn't discuss Brexit tonight. However, the basic principles and rules, namely the single market and indivisibility of the four freedoms, will remain our firm stance.”