The regulator of medicines is investigating whether there has been a systemic failure in the way healthcare products are repackaged as the Nurofen Plus recall widened to a police investigation into sabotage.

The Medicines and Healthcare products Regulatory Agency (MHRA) has highlighted concerns over the way in which left over packs of pills are recycled at pharmacies and wholesalers.

The manufacturer Reckitt Benckiser suspected foul play as it began working with police yesterday to establish how pain killing tablets were replaced by the anti-psychotic drug Seroquel XL.

The firm issued the recall of an estimated 250,000 packets in UK homes on Friday and stopped distribution after boxes were found containing other drugs.

There have been five reports of different batches of 32 tablet packets where blister strips were replaced with other manufacturer's medicines.

Four foil packs containing Seroquel XL 50mg were found in three Boots stores and an independent pharmacy in South London, while Pfizer's anti-epileptic drug Neurontin was found in 100mg capsules at an independent pharmacist in Northern Ireland.

Seroquel XL is a prescription-only drug used to treat disorders such as schizophrenia, mania and bipolar depression.

People who take Seroquel may experience sleepiness and are advised not to drive or operate any tools or machinery.

Two people are known to have taken Seroquel XL in error, but have not been adversely affected. No one has been recorded taking Neurontin from the packs. The drug may cause dizziness.

Medicines are routinely put into new packets when blister packs do not exactly match a GP's prescription, where a course requires a smaller number of pills than is used in standard boxes.

Last night the Department of Health confirmed three South London pharmacists involved were part of the Boots Group in Bromley, Beckenham and Victoria while a fourth was at an independent Pharmacist in Beckenham.

Reckitt Benckiser said Nurofen is manufactured in Nottingham where tablets are automatically packed into boxes on pallets and sent to distribution centres for despatch.

MRHA has not ruled out criminal tampering, but is also investigating whether pills could have been incorrectly repacked as part of "consolidation", where left over medicines are put in new boxes by pharmacies, or where pharmaceuticals are returned to wholesalers.

"Medicine consolidation [occurs] if you go to your pharmacist and you have a prescription for, say, six tablets and box might contain, for example, eight tablets. In that case the pharmacist might give you six and cut a blister down and take two tablets back," an MRHA spokesman said.

"The pharmacist can hold those tablets and reconsolidate them, and make them up into other packets and use them. That certainly has potential for a blister of Seroquel to get mixed in with Nurofen Plus tablets.

"We haven't ruled out tampering just as we have not ruled out consolidation or returns errors."

The MHRA is working with Reckitt Benckiser in the investigation and stressed returns and consolidation were two further lines of inquiry alongside sabotage.

In a briefing with The Independent on Sunday, the regulator advised it was unlikely that medicines could have been sent back to the factory where they were manufactured, adding that the mix of packets could have occurred in error at a wholesalers.

The investigation has almost entirely ruled out a manufacturing error as it was "not really feasible" there was a mix up at the factory because Seroquel maker AstraZenca uses a different manufacturing site.

A Reckitt Benckiser spokeswoman refused to be drawn on accusations of systemic failure involving consolidation or returns of medicines, adding that no lines of inquiry have yet been eliminated. "We are investigating fully and it's too early to speculate," she said last night.

The police said the Specialist Crime Directorate is now conducting an investigation after the incidents were referred to the Metropolitan Police Service on Friday.Boots UK said last night the firm "would not re-consolidate over the counter or prescription based medicines".

"As the UK's leading pharmacy-led health and beauty retailer, our customers' health and safety is of the utmost importance. If you are concerned about your Nurofen Plus, please return them to the pharmacy where they were purchased," a spokeswoman said.