By Sumeet Desai and Marc Jones

LONDON (Reuters) - The government has decided to nationalise Northern Rock, abandoning a five-month attempt to snare a private sector buyer for the ailing bank.

Britain's fifth-largest mortgage lender has borrowed 25 billion pounds from the Bank of England since its funding model collapsed in the credit crisis last year, sparking the first run on deposits at a British bank for some 140 years.

The Northern Rock NRK.L debacle has become a major headache for Brown and his Chancellor Alistair Darling, tarnishing the government's popularity and denting the prime minister's reputation for being a guardian of financial stability.

Darling told a hastily arranged news conference that nationalisation would be temporary and the bank would be returned to the private sector when markets stabilised. But it was the best option for protecting taxpayers.

"Market conditions will improve. Northern Rock's mortgage book is good but I think it would be a mistake for us to abandon this asset and take a loss now," he said.

"We had to intervene here, because if we let this bank fail there was every chance ... the problems would have spread into the wider British banking system," he said.

The mortgage lender already owes taxpayers 25 billion pounds and has been put on the government's books, classified as around 90 billion pounds of public debt.

The government will put forward legislation on Monday to take the bank into public hands -- the first major nationalisation in Britain since the 1970s -- and trading in Northern Rock shares was suspended.