Two British tourists have died in the Netherlands after snorting what is believed to have been white heroin that was sold to them as cocaine.
The men, aged 20 and 21, were found in an Amsterdam hotel room yesterday.
The Foreign Office is liaising with Dutch authorities and is "providing consular assistance to the family", a spokesman said.
Police spokesman Rob van der Veen said the bodies of the two men, both from England, were found next to traces of a white powder.
The men's names have not been released by officials.
A post-mortem examination is due to be conducted later this week to confirm the time and cause of their deaths.
"We did a quick test to find out what kind of powder it was. It wasn't cocaine, it was an opiate - in this case, white heroin," Mr van der Veen said.
"We took the sample to the Dutch forensic institute and they found out that the contents of this material is the same contents as other material that we already seized in other cases where people got sick."
He said investigators believe the men were targeted by the same street dealer, or ring of dealers, involved in the death of another Briton last month.
Local authorities have pleaded with tourists to steer clear of street pushers, and since October have distributed warning signs and flyers at hotels and hostels, main squares and at Amsterdam's Centraal railway station.
The pamphlets warn: "The last couple of weeks several tourists were hospitalized with severe health problems after using cocaine bought on the street ... Research has revealed that the cocaine used was not cocaine but white heroin."
They also remind visitors that in Amsterdam "you won't be arrested for taking drugs".