LONDON - Prime Minister Gordon Brown's popularity has plummeted after his government's reputation for competence was dealt a double blow, the latest opinion poll showed on Saturday.

The ICM poll in the Guardian newspaper, echoing earlier soundings, capped a bruising week for Brown whose administration came under fire over the Northern Rock banking debacle and the computer loss of almost half the population's personal details.

The "Brown bounce" -- he had contemplated calling a snap election after his popularity soared in September -- has vanished.

The poll showed just 31 percent backed Brown, down four percentage points from last month. It was the lowest ICM rating since he took over from Tony Blair in June after a decade waiting in the wings as finance minister.

The big winners in the poll were the centrist Liberal Democrats, the country's third biggest party, whose support rose three percentage points to 23 percent. The party is engaged in a contest to pick a new leader.

The Conservatives, the main opposition party, enjoyed the support of 37 percent but, worryingly for its leader David Cameron, their support was down three percentage points.

The ICM poll mirrored the sombre soundings delivered by a Populus poll on Friday in the Times newspaper.

It showed the share of voters who trust him and his finance minister, Alistair Darling, to handle the economy has fallen to just 28 percent since September.

The polls came after the government admitted computer discs containing details of 25 million people were lost in the post by Britain's tax authority.

An investigation has been launched and the head of the tax collection office has resigned over the largest data failure in British history.

It was the second scandal to hit the Brown administration in quick succession.

In September, Britain suffered its first bank run in 140 years after Northern Rock fell foul of the global credit crunch and had to borrow emergency cash from the Bank of England. The mortgage lender's shares keep sliding as it looks for a buyer.

Brown has also come under fire from five retired defence chiefs who in an unprecedented attack accused him of neglecting the armed forces and risking soldiers' lives.

He defended his government on Friday, saying he had a good record dealing with crises including floods, terrorism and an outbreak of foot and mouth disease.

Asked by reporters in Uganda, where he is attending a Commonwealth summit, if he was rattled by the polls, he replied: "I feel I'm getting on with the job".