The shirt’s manufacturer Nike has altered the appearance of the St George’s Cross using purple and blue horizontal stripes in what it called a “playful update” to the shirt ahead of UEFA Euro 2024.

Sunak warned Nike "should not mess" with the flag while Starmer urged Nike to "reconsider" the design.

The FA said it was "very proud" of the red and white St George’s Cross but gave its support to the new design.

"The new England 2024 home kit has a number of design elements which were meant as a tribute to the 1966 World Cup-winning team," a spokesperson said.

"The coloured trim on the cuffs is inspired by the training gear worn by England’s 1966 heroes, and the same colours also feature on the design on the back of the collar. It is not the first time that different coloured St George’s Cross-inspired designs have been used on England shirts.

“We are very proud of the red and white St George’s cross – the England flag. We understand what it means to our fans, and how it unites and inspires, and it will be displayed prominently at Wembley tomorrow – as it always is – when England play Brazil.”

The Prime Minister was asked about the kit design on a visit to Derbyshire.

“Obviously I prefer the original, and my general view is that when it comes to our national flags, we shouldn’t mess with them,” Sunak said.

“Because they are a source of pride, identity, who we are, and they’re perfect as they are.”

Labour leader Sir Keir Starmer called on Nike to reconsider, and told The Sun: “I’m a big football fan, I go to England games, men and women’s games, and the flag is used by everybody. It is a unifier. It doesn’t need to be changed. We just need to be proud of it.

“So, I think they should just reconsider this and change it back. I’m not even sure they can properly explain why they thought they needed to change it in the first place.”

A petition on change.org calling for a design change had already attracted more than 21,000 signatures by noon on Friday.

However, former national team star John Barnes could not understand what all the fuss was about.

“I don’t get involved in culture wars any more but this whole furore … I didn’t even know there was a St George’s cross,” he said.

“If they were going to change the three lions then that’s a debate to be had. I don’t see what the fuss is. I think it’s a much ado about nothing.

“They are not changing the colour of the shirt, the lions are still there. If they were going to change the national flag for England and change the colours then that’s a proper debate to have.”

Nike issued a statement insisting it “was never its intention to offend” with the design but did not indicate any intention to change it.

“We have been a proud partner of the FA since 2012 and understand the significance and importance of the St George’s Cross and it was never our intention to offend, given what it means to England fans,” the statement read.

“Together with the FA, the intention was to celebrate the heroes of 1966 and their achievements. The trim on the cuffs takes its cues from the training gear worn by England’s 1966 heroes, with a gradient of blues and reds topped with purple. The same colours also feature an interpretation of the flag on the back of the collar.”

The kit pricing has also been criticised.

An “authentic” version costs £124.99 ($242) for adults and £119.99 ($230) for children while a “stadium” version is £84.99 ($164) and £64.99 ($126) for children.