Furious parents have accused a Wimbledon prep school of giving its pupils an unfair advantage in important exams.

Fee paying Donhead school gave its year six children practice for Cognitive Ability Tests (CAT), used to determine the ability of all boys joining Wimbledon College in September.

Father Adrian Porter, headteacher at Wimbledon College in Edge Hill, is understood to be unhappy about the practice, which has taken place for at least two years, but has allowed the test results to stand.

One mother from Sacred Heart Primary School, who asked not to be named, said that the news had outraged parents from other schools whose sons had not been prepared for the test.

She said: "We can't believe that a school would do this. We feel the Donhead children should be retested - once you get into a set its very difficult to get moved up."

The advantage for Donhead boys was discovered during Saturday's exam, when parents from the prep school let slip that their sons had practised the exam in class the previous Tuesday and even brought practice papers home to work on with their parents.

Irene Pinner, the headteacher at Sacred Heart, said that the incident had upset parents and pupils.

"The children and parents feel there has not been a level playing field with the CAT test," she said.

She also confirmed that Wimbledon College had not said the test was available online to be practised beforehand, or that other children taking the test would have seen similar papers in advance.

CAT exams assess the ability of children to think using numbers, words and shapes.

Donhead headteacher Chris McGrath said the school used the tests to measure the progress of all its pupils, not just those heading to Wimbledon College, but denied coaching them.

"It's part of an assessment we use for all pupils. The boys take the test but receive no feedback," he said.

He also said that CAT tests could be bought by all schools and were widely used.

He declined to comment on the claim that Father Porter had asked Donhead, which is also based in Edge Hill, to end the practice.

Wimbledon College would not discuss the story.