UPDATE 1-Brown tries to buoy support for UK Afghan mission

* Poll shows backing for British involvement still firm

* Government rejects criticism of equipment failings (Updates with opinion poll paragraphs 3 and 4)

By Adrian Croft

LONDON, July 12 (Reuters) - British Prime Minister Gordon Brown's government tried to shore up public faith in the Afghan war effort on Sunday after the death of eight British soldiers in 24 hours sowed doubts over the mission.

The number of British troops killed in Afghanistan -- 184 -- now surpasses the British toll from the Iraq conflict.

However, an opinion poll conducted for the Guardian newspaper and the BBC showed support for the war remained firm.

The ICM survey put backing for British involvement at 46 percent, slightly behind opposition at 47 percent and up 15 points from the last time opinion was tested in 2006.

Brown said a major British and U.S. offensive against Taliban insurgents in Afghanistan's southern Helmand province was succeeding despite the heavy toll.

Taking his message direct to the troops, Brown told the British Forces Broadcasting Service: "I know that this has been a difficult summer so far and it is going to continue to be a difficult summer."

Asked if he was worried that the Helmand operation could become Britain's Vietnam, Brown said: "The operation ... is showing signs of success."

The operation is aimed at making Helmand safe for people to vote in Aug. 20 presidential elections.

SHORTAGE OF HELICOPTERS

Conservative former Defence Secretary Tom King told the BBC on Sunday British forces suffered from a critical shortage of helicopters and had never had enough troops "to do the job".

The government denies such criticism. Finance minister Alistair Darling said on Saturday British troops in Afghanistan would get whatever equipment they needed.

Britain has 9,000 soldiers in Afghanistan, the second largest foreign contingent after the United States.