The 25 words listed here are among the most common and the first acquired when learning to speak.

Professor Leslie Rescorla, who designed both versions of the test, said: ‘If children don’t use most of these words by 24 months, they may be late talkers.’

Many late-talkers are simply late bloomers, so the professor says that if a child is otherwise developing normally, parents shouldn’t panic.

But if the child is still struggling for words by two and a half, they should consider help such as speech therapy, and certainly not put this off past the age of three.

The Vancouver conference heard that up to 20 per cent of all two-year-olds are behind their peers in speech.

Of these, half to a three-quarters are likely to be late-bloomers, who will more or less catch up over time.

But in other cases, the child will have lasting problems with speech.

A small vocabulary at the age of two can also be a sign of other problems, from deafness to autism and dyslexia.

Even those who do catch up may always be slightly behind.

Professor Rescorla, of Bryn Mawr College in Pennsylvania, tracked the fortunes of 78 two-year-olds for 15 years.

Half had been slow to start talking but didn’t have any other problems.

By the time they reached 17, their vocabulary was classed as at least as good as average - but still wasn’t as good as those who were better talkers as toddlers.

The late-talkers fared particularly poorly in tasks that involved ‘verbal memory’, or listening to words, sentences, or numbers and being able to repeat them back

The conference also heard that TV and videos, also known as ‘overheard speech’, are no substitute for parental attention.

Real interaction with the child is particularly crucial when the child is trying to ‘crack the code’ of speech.