An A-level physics paper, which has prompted a flurry of complaints on social media, had a mistake in a question, admits the AQA exam board.

Four schools have already been in touch with the exam board about the A-level paper taken on Thursday.

A statement from the exam board says examiners will take the mistake into account when marking.

Students on social networking websites had complained that the question had been impossible to answer.

The AQA exam board denied this, saying that the question could still be answered, despite the error in how the question was presented.

"At this stage it looks as though most students haven't been affected by this, but we're letting our examiners know and will make sure it doesn't affect students' grades," said an AQA spokeswoman.

"We meant to ask students to compare a 150 kΩ resistor - not a 150 Ω resistor as it said in the paper - to a 300 kΩ resistor. The question can still be answered."

The exam board believed that students in about 1,200 schools and colleges would have been taking the paper.

Daniel Ohrenstein, a pupil in north London who was taking the exam, said a few lost marks could be the difference in missing out on a university place.

He said that it added to the stress of taking A-levels.

Pupils facing a question which seemed wrong would still "give the benefit of the doubt" to examiners and assume it was correct, he said.

That meant that students could waste time trying to make sense of a rogue question.