The Treasury expects Northern Rock to lose retail deposits worth billions of pounds once the Government removes financial guarantees from the state-owned lender in the spring.

The Times has learnt that the Treasury is assuming that a significant number of savers will rush to flee the bailed-out bank once the Government withdraws guarantees that protected 100 per cent of deposits.

According to Treasury insiders, the Government intends to remove the 100 per cent guarantees in the spring so that the so-called “good” bank can be valued and sold back to the private sector.

According to the mortgage bank’s third-quarter figures, released on Wednesday, Northern Rock has about £19 billion worth of retail deposits on its books.

In the event that Treasury assumptions are realised, the exodus is bound to lower the price that the Government could attract in any sale of the good bank of Northern Rock.

Gordon Brown wants to be able to say to voters that one of the bailed-out banks is well on the way to being sold back to the private sector, securing a return on taxpayer rescue funds.

To prepare Northern Rock for a sale, the Treasury has been forced to slice the lender into two parts.

The “good bank” will house about £10 billion worth of mortgages and £19 billion worth of deposits, while the “bad bank” will contain the remaining £55 billion worth of mortgages.

Only the good bank will be sold to the private sector, while the bad bank will remain within government hands and run off until the term of existing mortgages has expired. There will be no new lending within the bad bank.

The process to carve up the bank is proving to be so complicated that the Treasury has ruled out a sale before the general election, expected in May.

A spokesman for Northern Rock yesterday insisted that there was no sale timetable and no schedule as yet for the date when the Government’s 100 per cent guarantees would be lifted.

But he explained that the lender would have to give a minimum of three months’ notice to remove the government guarantees and that once they were removed, Northern Rock’s savers would be protected by the Financial Services Compensation Scheme, which guarantees up to £50,000 worth of savings for every investor.

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