A synthetic opioid that is 100 times more powerful than heroin is being added to batches of the street drug and is thought to have claimed the lives of at least six addicts in recent weeks.

Fentanyl - which was blamed for the death of the pop star Prince year - and carfentanyl has been detected in heroin that is being sold in the north of England.

The National Crime Agency and West Yorkshire Police raided a laboratory suspected of producing fentanyl and carfentanyl on Monday.