Thousands of Nurofen Plus packets may contain strong anti-psychotic pills used to treat schizophrenia, manic behaviour and depression and bi-polar disorder. Sabotage has not been ruled out.

Last night, medicine watchdogs issued an urgent warning after three Nurofen Plus painkiller packets were found to contain anti-psychotic Seroquel XL.

The faulty packets came from three different batches bought in London. Each batch contains around 5,000 boxes, meaning half a million tablets may be mixed-up.

Seroquel XL – available only on prescription – can cause extreme drowsiness or dizziness.

People who find Seroquel XL in Nurofen Plus packs have been told to contact their local pharmacy or GP or call the Medicines and Healthcare products Regulatory Agency’s hotline on 020 3080 6574.

Reckitt Benckiser, manufacturer of Nurofen Plus, said investigations are under way to establish how the errors occurred.

A statement said: “After careful review of the manufacturing system, errors by the makers of Nurofen Plus or Seroquel XL are not thought to be part of the cause.”

However, Sian Boisseau, director of independent health care consultancy Virgo Health, said:

“There has been a suggestion that the packets were deliberately put in the wrong boxes.” The large Seroquel XL 50mg capsules have gold and black packaging while the Nurofen Plus tablets are smaller and have silver packaging.

The mix-up is made more improbable by the fact that Seroquel XL is manufactured by another firm, AstraZeneca.

All affected pills were bought in south London.