An American scientist has sparked a trans-Atlantic tempest in a teapot by offering Britain advice on its favourite hot beverage.

Bryn Mawr College chemistry professor Michelle Francl says one of the keys to a perfect cup of tea is a pinch of salt.

The tip is included in Francl’s book Steeped: The Chemistry of Tea, published Wednesday by the Royal Society of Chemistry.

Not since the Boston Tea Party has mixing tea with salt water roiled the Anglo-American relationship so much.

“Pardon,” wrote popular brand Yorkshire Tea on X – in something of a masterful display of British understatement.

Among the responses to its tweet was this one: “I’m not taking culinary advice from the country which made cheese in a can.”

Indeed, the salt suggestion has also drawn howls of outrage from tea-lovers in Britain, where popular stereotype depicts Americans as coffee-swilling boors who make tea, if at all, in the microwave.

“Don’t even say the word ‘salt’ to us …” the etiquette guide Debrett’s wrote on X, formerly known as Twitter.

Tweet from @USAinUK

The US embassy in London intervened in the brewing storm with a social media post reassuring “the good people of the UK that the unthinkable notion of adding salt to Britain’s national drink is not official United States policy”.

“Let us unite in our steeped solidarity and show the world that when it comes to tea, we stand as one,” the tongue-in-cheek post said.

“The US Embassy will continue to make tea in the proper way – by microwaving it.”

Tweet from @UKinUSA

That shocked Britain’s Cabinet Office into an official response.

“We appreciate our Special Relationship. However, we must disagree wholeheartedly,” it wrote.

“Tea can only be made using a kettle.”

Steeped is the product of three years’ research and explored the more than 100 chemical compounds found in tea and “puts the chemistry to use with advice on how to brew a better cup”, its publisher said.

Francl said adding a small amount of salt – not enough to taste – made tea seem less bitter because “the sodium ions in salt block the bitter receptors in our mouths”.

She also advocated making tea in a pre-warmed pot, agitating the bag briefly but vigorously and serving in a short, stout mug to preserve the heat.

And she said milk should be added to the cup after the tea, not before – another issue that often divides tea-lovers.

Francl has been surprised by the level of reaction to her book in Britain.

“I kind of understood that there would hopefully be a lot of interest,” she told the Associated Press.

“I didn’t know we’d wade into a diplomatic conversation with the US embassy.”

– with AAP