The number of four-year-olds able to write their names when they started school doubled between 1997 and 1998, according to new research.
Parents and nurseries are thought to be training the under-fives in the new compulsory baseline tests, say the researchers from Durham University.
The team analysed 35,000 test scripts devised as part of Durham's Performance Indicators in Primary Schools (PIPS) project. They found that more than one in four children could write their name almost perfectly when they started school last year, compared to just over 10 per cent of the previous cohort.
Nearly three-quarters of four-year-olds who started school last September could recognise the first letter of their name, compared with 58 per cent last year, the Durham team found.
Mandatory baseline assessment was introduced in September and requires reception teachers to assess each child against goals which include counting to 10, writing their name and knowing the alphabet. It enables teachers to assess what skills each new child has and is intended to give them a way of measuring "value added" in primary schools.
Many schools have voluntarily used one of 91 accredited baseline assessment schemes, including the PIPS test, for several years.
Peter Tymms, director of Durham's curriculum, evaluation and management centre which carried out the research, said: "This is a surprisingly large and potentially very significant change to find in the space of one year. It really does look as if people have been sitting down with kids and teaching them to write in a way that has never happened before.
"The increased emphasis on reading and writing in primary schools now seems to have filtered down into nurseries. There was a lot of publicity when these children started school about baseline assessment so it would not be surprising if parents had caught wind of it ."
Wendy Scott, chief executive of The British Association for Early Education, said: "It is unsurprising but regrettable that once more it seems as though the tests are influencing the curriculum rather than the other way around.
"It is human nature for parents or nurseries to want children to do well in the tests but these short- term measures are not necessarily best for children in the long run."