Students are failing to spot the difference between simple words such as 'their' and 'there' or 'cloths' and 'clothes' amid confusion over the English language, said Vineeta Gupta, head of children's dictionaries at The Oxford University Press (OUP).
The students aged seven to 13 even fail to spell simple words such as 'surprise', 'excitement', 'weird', 'doesn't' and 'minute' correctly.
The OUP found that children in primary and secondary schools were increasingly encouraged to look up complex words using dictionaries and electronic spellcheckers, The Telegraph reported.
An analysis of more than 33 million words written by students of age group seven to 13 found that they regularly identified the correct spelling for terms such as 'pterodactyl' and 'archaeologist'.
However, too many pupils were falling down when presented with more common words.
In most cases, they failed to pick out silent letters or the difference between a single or double letter in words such as 'disappeared' or 'tomorrow'.
The top spelling error was "accidentally", followed by 'practising','frantically', 'definitely' and 'believe', it was revealed.
The research comes after the introduction of a new spelling test by the UK Government.
For the first time this year, all six-year-olds have been given a new assessment using phonics - the back-to-basics spelling method that breaks words down into individual sounds.
But the OUP suggested that children were still being left confused by more unusual spellings in common words.
"Children are keen and motivated to spell well, and it is pleasing to know that they probably look words up that are technical or more complex," Gupta said.
"At the same time, children are still struggling with simple and everyday words. Spellcheckers can be useful but may not provide all the support a child needs to distinguish confusables such as their/there and cloths/clothes," Gupta added.
Researchers analysed pupils' spelling skills using the Oxford Children's Corpus - a database containing the authentic written work of almost 75,000 children.
