Just half of the 306 seven to 11-year-olds spelt "their" correctly while one in three got "course" right. Many struggled with words like "could", "might" and "too".
The reason for the poor performance according to Ken Spencer of Hull University's Institute for Learning, lies with the inconsistent way English is spelt.
He says English pupils are at a disadvantage compared with German and Italian children whose languages are spelt more consistently.
German and Italian children can look at a text and work out the words, but English-speaking children need help to "decode" it.
On the basis of Dr Spencer's tests, held in an average Hull primary school, he put words into three groups. About a third of them, including "and", "the" and "good", were spelt correctly by between 90 and 100 per cent of pupils and rated easy. A further third, including "how", "now" and "said", were spelt correctly by 80 to 90 per cent of pupils and rated moderate. But a final third, misspelt by between 34 and 80 per cent of pupils, were rated difficult.
Dr Spencer found the relative difficulty of words had less to do with frequency or length than with the consistency with which the sounds were represented in writing.
To help children, Dr Spencer has developed "Simpl Inglish", a representation of all 44 sounds in the English language using the 26 letters of the alphabet, based on the ideas of "pinyin" in Chinese. Combined sounds like "th" have a circle round them; long vowel sounds are marked with a circumflex.
Children learn to read using this Inglish, then have the standard form of the words written above or below and learn to "bridge" from one system to another.
Dr Spencer has tested the system in one-to-one tuition with primary pupils in Hull who have difficulties with reading and says it has achieved remarkable improvement.
His study showed how the difficulty of English spelling opened up a gulf in achievement between the more and less able.
Two-thirds of the least-able seven-year-olds could spell the easy words correctly, but only 10 per cent got the difficult words right. Two years later, the least-able nine-year-olds had reached the same level of proficiency on the moderate words as the seven-year-olds had reached on the easy words.
Even after three years, the least-able 10-year-olds were still having more problems with the difficult words than the average seven-year-old with the easy ones.