If you tell us what they say instead then I can probably give you some insight.

There's "bovvered" or there's "what da hell" or there's "fankyou"...

I can probably think of several more dialects where "th" is replaced with something else.

There's nothing sudden about it. Many regional (mostly city) accents have common mis-pronounciation. "th" being mis-pronounced as either "f" or "v" was extremely common in the area I grew up 20+ years ago, and I'm sure it predates me.

I'll take that any day over some of what passes for dialects in the US.

What are they saying instead of th, when did this start, and is it particular parts of Britain or particular social groups?

(Using f/v instead of th has long been a feature of some London dialects, and using t/d instead of th has long been a feature of some Irish dialects, and young children often use f/v instead of th, I think. Jocular references to "the youf" have been around in print for a while.)

EDIT: Of course there's also Jamaican pronunciation with t/d instead of th!

As an aside, the dialect I used in my youth used "f" a lot - fit, fa, fan and far for what, who, when and where. Leading to "Fit fit fits fit fit" making sense...

North east?

Yes, from a village near Buckie. Which would probably be "Fae aside Buckie".

Thanks, that's about as north east as one could get!

There is a technical linguistic term for it - th fronting:

en.wikipedia.org/wiki/Th-fronting

Apparently it's been around for quite a bit of time.

I believe this is a sign of a hearing problem. Some children are unable to hear a strong difference between certain sounds so, to them, they are speaking the same as everyone else. It used to be that those children would be identified and given special help to allow them to hear better. I remember having eye and ear tests at school. Maybe this has stopped now?

The tragedy is that this is a huge dividing factor in the UK. In all the places I've worked the vast majority of people use the distinct "th" sounds. Use of "f" and "v" sounds is, to use the current terminology, a "red flag".

Doesn't seem likely to me - it's common with specific strong dialects and often associated with working class people. As it's regional, I don't see it being a hearing related issue (not that I think it's an issue anyhow).

I agree. Most speakers could probably switch to any manner of accent at will.

You mean a red thlag?

That's classism, Britain is a very classist country.

Why have Americans always been unable to pronounce the intervocalic T?

Bread and budder? WTF?

I'm an author; I love horseback writing?

I'm tired of wading for the flood?

I want to ride my modercycle?

It's so pervasive that we really ought to update our dictionaries to replace these, now phantom, "t"s with proper "d"s. Think of it as antipsychotics for a batshit crazy language.

I'm American. But this has always been annoying, along with most of the eN language.

To cast a cynical eye on the bright side; it seems both the "t" and "d" will soon be obsolete as the entire eN language is refined to tonal variations of our most common word/punctuation/expression/noise, "like".

I may be mistaken, but I believe there's evidence that new humans are forming a "like-gland". The sound will become as natural and necessary as that of frogs bleating* after rain.

*Yes, bleating, not bleeding.

Reminds me of going into a shop while visiting the U.S. and trying to buy some batteries. The employee struggled with my english accent, so I had to try to explain what batteries where. He eventually understood and exclaimed "oh, badderies!".

I highly doubt the t's were the issue. Different stress, more pronounced articulation/less syllable smoothing, and vowel pronunciation are more likely culprits. It's very hard for the speaker to hear, but very noticeable to everyone else (seen this many times trying to get people to pronounce Finnish words like sauna or Helsinki correctly, they can't hear the difference).

I've got a friend with a story from his high school job at a retail store, about his having to translate for the other workers so they could figure out WTF this British guy was asking for.

"Batcheries"

The t can also disappear. I've heard people say "twenny" instead of "twenty". Perhaps because "twendy" sounds more like Elmer Fudd giving Bugs Bunny decoration advice.

It's from Scandinavian immigrants influencing the accent. It's why Minnesota is pronounced with a "d" sound.

> Why have Americans always been unable to pronounce the intervocalic T?

> Bread and budder? WTF?

In this and most cases there isn't really another word that would fit in context. Also plenty of other dialects (i.e. Australian English) do exactly the same thing.

> I'm an author; I love horseback writing?

I'm this case, "riding" and "writing" have a marked difference in the length of the initial diphthong sound "ai".

I general American English is nowhere near the most ambiguous in this way. Many languages rely more heavily on context because they use ambiguous phonological and grammatical marking in common speech (i.e. spoken Japanese).