German Chancellor Angela Merkel has expressed "grave concern" to Boris Johnson about allowing large crowds to watch the Euros at Wembley.
Mrs Merkel questioned the decision to allow tens of thousands of spectators into the stadium for the semi-finals and the finals next week amid fears over the spread of the Delta variant.
Crown capacity will be increased to more than 60,000 fans under the UK Government's pilot scheme for return of crowds to large events.
The move was given the green light despite Mr Johnson's decision to delay the final stage of lockdown lifting from June 21 to July 19.
The German leader publicly questioned the wisdom of the plan at a joint press conference with Mr Johnson at his Chequers retreat on Friday.
"I say this with grave concern. I have also said this to the Prime Minister," she told reporters.
"We in Germany, as you know, have less people attend games in the Munich stadium but the British Government will obviously take its own decision.
"But I am very much concerned whether it is not a bit too much."
But Mr Johnson rebuffed her calls to reduce spectator numbers, which will put the stadium at 75% capacity for the first time since the pandemic began.
"Of course we'll follow the scientific guidance and the advice if we receive any such suggestion," he said.
"But at the moment the position is very clear in the UK, which is that we have certain events which we can put on in a very careful and controlled manner with testing of everybody who goes there.
"There in the UK we have built up a very considerable wall of immunity against the disease by our vaccination programme."
He insisted there had been "a big change" in the relationship between case rates and levels of serious illness and death since the start of the jabs rollout.
"I want to stress that we have been very cautious at every stage. But that's why I think it's been effective and it's why it's been an irreversible road map," he said.
Scientists behind the Government's Events Research Programme found only 28 Covid cases and no major outbreaks in the first phase of pilots.
These included the FA Cup Final and the Brit Awards earlier this year.
Fans attending the Euros matches must follow strict rules, including providing proof of a negative Covid-19 test or proof of full vaccination.
