There is little that irks British defenders of the English language more than Americanisms, which they see creeping insidiously into newspaper columns and everyday conversation. But bit by bit British English is invading America too.
"Spot on - it's just ludicrous!" snaps Geoffrey Nunberg, a linguist at the University of California at Berkeley.
"You are just impersonating an Englishman when you say spot on. Will do - I hear that from Americans. That should be put into quarantine," he adds.
And don't get him started on the chattering classes - its overtones of a distinctly British class system make him quiver.
But not everyone shares his revulsion at the drip, drip, drip of Britishisms - to use an American term - crossing the Atlantic.
"I enjoy seeing them," says Ben Yagoda, professor of English at the University of Delaware, and author of the forthcoming book, How to Not Write Bad.
"It's like a birdwatcher. If I find an American saying one, it makes my day."
Last year Yagoda set up a blog dedicated to spotting the use of British terms in American English.
So far he has found more than 150 - from cheeky to chat-up via sell-by date and the long game - an expression which appears to date back to 1856, and comes not from golf or chess, but the card game whist. President Barack Obama has used it in at least one speech.
Yagoda notices changes in pronunciation too - for example his students sometimes use "that sort of London glottal stop", dropping the T in words like "important" or "Manhattan".
Kory Stamper, associate editor for Merriam-Webster, whose dictionaries are used by many American publishers and news organisations, agrees that more and more British words are entering the American vocabulary.
Stamper is one of the powerful few who get to choose which words are included in the dictionary, as well as writing their definitions.
One new entrant into the Merriam-Webster dictionary in 2012 was gastropub (a gentrified pub serving good food), which was first used, according to Kory Stamper, in London's Evening Standard newspaper in 1996, and was first registered on American shores in 2000.
"The British pub is a very different critter from an American bar," she says, but bars with good beer and food are springing up in many cities in the US, and the British term is sometimes used to describe them.
Twee (excessively dainty or cute) is another "word of the moment", says Stamper, as is metrosexual (a well-groomed and fashion-conscious heterosexual man) which "took off like wildfire", after it was used in the American TV series Queer Eye. There was even a backlash against it - a sure sign, she says, that the word had "absolutely made its way into the American vernacular".
There has also been "a huge up-tick", says Stamper, in the use of ginger as a way of describing someone with red hair.
She sees this as clearly tied to the publication in the US of the first Harry Potter book. Dozens of words and phrases were changed for the American market, but ginger slipped through, as did snog (meaning "to kiss amorously") - though that has not proved so popular.
We are not seeing a radical change to the American language, says Jesse Sheidlower, American editor at large of the Oxford English Dictionary - rather a "very small, but noticeable" trend.
Bill Kretzschmar, professor of English at the University of Georgia, makes a similar point - that while the spike in use of some British terms may look dramatic, it is often because they are rising from a very low base. Most are used "very infrequently", he says.
And it is not so much the masses who use these terms, says Geoffrey Nunberg, as the educated elite. Journalists and other media types, like advertising agencies, are the worst offenders, in his view.
"The words trickle down rather than trickle up," he says.
"It sounds trendy - another borrowing we could use without - to use a British term. It just sounds kind of Transatlantic."
But the line between trendy and plain pretentious is a fine one, says Sheidlower.
Anyone who says bespoke - as Americans sometimes do when referring to a custom-made suit or a bicycle - is just "showing off".
But some British terms can be useful, says Sheidlower, and fill in a gap where there is no direct equivalent in American English - he cites one-off (something which is done, or made, or which happens only once) as an example.
To go missing is another useful term, says Ben Yagoda, as it is more nuanced, conveying a greater sense of uncertainty than the standard "to disappear". Its use climbed significantly in 2001, with the high-profile case of the missing intern Chandra Levy.
British TV shows like Top Gear, Dr Who, and Downton Abbey may be another reason more British words are slipping in, says Yagoda, as well as the popularity (and easy access via the internet) of British news sources, such as The Guardian, The Economist, The Daily Mail - and the BBC.
Yagoda also points to a number of British journalists who have risen to influential positions in the US, including Tina Brown - who has worked as editor of Vanity Fair, The New Yorker, The Daily Beast, and Newsweek - and Anna Wintour, editor in chief of American Vogue.
"English for everybody is becoming more international, every day that passes," says Bill Kretzschmar who is also editor in chief of the Linguistics Atlas Project, which tracks spoken English.
The use of university, rather than college or school, for example, may well be used by Americans to make sure they are understood outside the country.
The same thing might be influencing a trend that Yagoda has spotted for Americans to use the day, month, year format for dates - 26/9/12 rather than 9/26/12.
There is not so much an "on and off switch" between versions of English, says Kretzschmar, but more of a continuum - with the same words in existence in different places, but just used at different frequencies.
Some words, often the more formal ones, were once common on both sides of the Atlantic, but dropped out of American English usage while remaining popular in Britain, says Yagoda - amongst (instead of among), trousers (instead of pants), and fortnight (two weeks) are examples.
And some words which Brits regard as typically American - including "candy", "the fall", and "diaper" - were originally British, but dropped out of usage in Britain between about 1850 and the early 1900s, says Kory Stamper.
"America has always welcomed words from all over," she says.
"If it doesn't look conspicuously foreign, I don't think anyone questions - it's just English at that point."
The word gormless (the best American equivalent is probably "clueless") is on the rise in the US, for example, says Stamper, but no-one thinks of it as a British word. For some reason it sounds Southern to many American ears.
There would have been no difference between British and American English when the Pilgrim Fathers first crossed the Atlantic. It took time for the two to go their separate ways - a process given a jolt by Noah Webster, who published the first dictionary of American English in 1806, 30 years after the Declaration of Independence.
Webster introduced the distinctive American spellings of words like "honour" (honor), "colour" (color), "defence" (defense), and "centre" (center), as well as including specifically American words like "skunk" and "chowder".
"He wanted very much for this budding new nation to have its own language," says Kory Stamper, whose Merriam-Webster dictionary is the modern-day version of Webster's work.
"If [we were] not British, but American, we needed to have an American language as well."
These days, the "balance of payments" language-wise is very much skewed the other way - with Americanisms used far more in Britain than the other way round, says Nunberg.
And though a few people do take umbrage at the use of British words in American English, they are in the minority, says Sheidlower.
"In the UK, the use of Americanisms is seen as a sign that culture is going to hell."
"But Americans think all British people are posh, so - aside from things that are fairly pretentious - no-one would mind."