Drugs regulators have launched an investigation after packs of the painkiller Nurofen Plus were found to contain antipsychotic drugs.

The Medicines and Healthcare products Regulatory Agency (MHRA) issued a safety alert following reports that some batches of Nurofen Plus contained individual blister packs of another drug, Seroquel XL 50mg.

Packs from the three batches -- numbered 13JJ, 57JJ and 49JJ -- have been distributed across the UK and thousands could be affected.

It is unclear how the mix-up could have happened, especially as Seroquel XL is manufactured by AstraZeneca, while Nurofen is made by Reckitt Benckiser. Neither company is ruling out the possibility of sabotage, media reports said.

The MHRA warned anyone buying the painkillers to be extra vigilant, while pharmacists have been advised to check before selling packs of the over-the-counter drug.

The large Seroquel XL 50 mg capsules can be identified by their gold and black packaging while the smaller Nurofen Plus tablets have silver and black packaging.

Ian Holloway, from the MHRA's defective medicines report centre (DMRC), said: "People should check to see if they have any affected packets of Nurofen Plus. If you do, return them to the pharmacy where you bought them from. You can also report this to the MHRA?s DMRC on 020 3080 6574.

If you have taken a tablet and you have any questions, speak to your GP.

Reckitt Benckiser said there had been three cases identified so far, all within the south London area. Thousands more packets could potentially be affected.

"Serious investigations" are under way to establish how the mix-up occurred, a statement issued by the company said.

"After careful review of the manufacturing system, manufacturing errors by the makers of Nurofen Plus or Seroquel XL are not thought to be part of the cause at this stage.

"We are taking this matter very seriously and are working closely with the regulatory authority, the MHRA, and pharmacies.

"The MHRA are investigating the issue, and have considerable law enforcement powers."

Seroquel XL is a prescription-only antipsychotic drug used to treat several disorders including schizophrenia, mania and bipolar depression, while Nurofen Plus is a pain relief drug containing codeine and ibufrofen.

The three batches of the painkiller affected each contain between 4,000 and 7,500 packs, however not all the 32-tablet packs were affected.