As a result, police are urgently appealing to more than 20,000 clubbers who attended dance parties at Alexandra Palace in North London at the weekend to seek immediate medical attention if they had taken similar drugs.

A third man is in a serious condition in hospital and doctors have reported at least 20 similar cases across the capital and Home Counties.

Scotland Yard detectives suspect they may have all consumed doses of MDMA, the chemical name for ecstasy.

Both victims died within seven hours of each other yesterday morning after being taken to the Whittington Hospital in North London. The first, aged 21, attended an all-night party called Bass Culture, which started on Friday and continued until 6am on Saturday.

The second victim, aged 20, went to the Epic event featuring veteran BBC DJ Pete Tong which began on Saturday evening and ran until Sunday morning.

The deaths come just a week after the charity Drugscope, which monitors trends in underground the drug trade, warned of an alarming rise in the popularity of ecstasy which dominated the 1990s rave scene but fell out of fashion.

A surge in use follows an influx of a more-potent Chinese variants of the drug which is based on the chemical MDMA, or methylenedioxymethamphetamine.