Eighteen sailors on a Royal Navy warship have tested positive for a class A drug, the Ministry of Defence (MoD) said today.

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The crewmen were caught during routine testing onboard HMS Liverpool, which is currently deployed in the South Atlantic.

An MoD spokeswoman said the drug involved was cocaine and internal disciplinary action was being taken against the 18.

The drug bust is believed to be the biggest in the Navy's history.

It comes after five soldiers from the King's Troop Royal Horse Artillery were dismissed for taking cocaine - and eight from the 2nd Battalion of the Royal Regiment of Fusiliers were found to have traces of cocaine and cannabis during a compulsory drug test in July.

HMS Liverpool is a Type 42 destroyer whose duties in the South Atlantic have included combating drug smuggling.

The MoD spokeswoman said the test had been carried out after the crew had a "run ashore" on a break in Brazil.

She said: "The Royal Navy does not tolerate misuse of drugs by its personnel and internal action is underway against all 18 individuals.

"Notwithstanding the numbers involved in this single unprecedented incident which is being treated very seriously, it has not affected the ship's ability to do its job.

"These results must be understood in context. Tests show that drug misuse is almost 20 times more common in civilian work places than it is in the Navy. Positive test rates in the Navy last year averaged less than 0.4 per cent, compared with over 7 per cent in civilian workplaces.

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"We are not complacent though and our compulsory drug tests will continue to expose those few that let the rest down."

She continued: "The UK Ministry of Defence conducts Europe's largest compulsory drug testing programme and this has significantly reduced drug misuse among service personnel."

A senior Royal Navy source told The Sun: "The worst aspect of this scandal is that HMS Liverpool is supposed to be guarding the Falkland Islands and such a large drugs bust hardly does wonders for confidence.

"The idea of a coked-up rating on board a warship with access to such powerful weapons does not bear thinking about. This is a total embarrassment for the senior service."