"Or to put it another way, the nationalised bank is having a mega rights issue that taps its one shareholder," BBC business editor Robert Peston said.
"It is similar to what other banks have been doing recently, where they have been having rights issues.
"But the £3bn is taxpayers' money - that is money that is very much at risk."
'State aid approval'
Chancellor Alistair Darling told Radio 4's Today programme that Northern Rock, like other banks, found itself in a "very difficult situation".
The Liberal Democrats' Vince Cable calls for legal action against the private directors
"Northern Rock needs more share capital," he said.
"It doesn't have shareholders now that... it's owned by the government, so it's got to come to us, so what we've said is that we will put in up to £3bn.
"We've got to get state aid approval for that so we can't give a precise figure, but up to £3bn may be necessary."
The Conservative's Treasury spokesman Philip Hammond said the taxpayer "was being forced to hand over yet more money in order to keep this bank afloat".
And Liberal Democrat Treasury spokesman Vince Cable said: "Alistair Darling assured Parliament that taxpayer loans to Northern Rock would be fully secured on mortgage assets. This is clearly not true."
Newcastle-based Northern Rock revealed that the total percentage of its residential mortgage customers in arrears and being repossessed had risen sharply from last year.
If the government tried to sell the Rock into the private sector right now, the loss would be colossal
In June 2007 the figure stood at 0.38% of mortgages, but it has now risen to 1.18%. That however, is lower than the Council of Mortgage Lenders average of 1.21%.
The company said this is due to more difficult economic and market conditions.
The number of house repossessions has also risen from 2,215 to 3,710 - an increase of 67%, which represents 0.56% of all mortgage accounts.
The company says this is because it is moving to repossess properties more quickly when it is clear that customers will not be able to maintain their mortgage payments.
In a further sign of customers finding it harder to make ends meet, it also said it was increasing staff numbers in debt management from 185 to around 500.
Northern Rock had advanced some loans worth as much as 125% of home values, to some customers.
However arrears among those "Together" mortgage holders have soared from 0.73% a year ago to 2.14% this June.
Many of those customers could be left in a position of negative equity - where the value of the mortgage exceeds the value of their house, and may also struggle to find other sources of finance when their current loans end.
Northern Rock was hit hard by the global credit crunch, when its main method of financing mortgage loans - by borrowing cash from the wholesale markets - dried up.
The bank, once Britain's fifth-biggest home loan provider, was taken into public ownership after it failed to find a suitable buyer from the private sector.
In another sign of its scaling back, Northern Rock has reduced the number of its mortgage accounts by 15%, to 662,000 at the end of June compared to 777,000 at the end of 2007.
Loans and advances to customers have also been cut back - by £14.5bn in the first half of the year to £84.4bn.
Northern Rock's executive chairman Ron Sandler said he was pleased with the progress that had been made in repaying its debt.
"Repaying the government loan is proceeding well ahead of target," he said. "This is the result of the success we are having with our redemption plan."
Things could be a lot worse... 7,000 people could be out of work
The bank wants to shed 60% of its mortgage customers in the next couple of years.
The money that is redeemed will be used to repay all the government's emergency lending by the end of 2010.
Mr Sandler also said the housing market would "look very soft for some months in the future".
He added:"The external environment has deteriorated and the consequences of this for Northern Rock are increased credit losses."
Northern Rock said last week that it was to cut 1,300 jobs as it slims down its business.
In its results it said it was facing a cost of £37m associated with redundancy, payments in lieu of notice, and outplacement services as part of the restructuring of the business.
Mr Sandler would not reveal the size of redundancy packages but said the payments would be "fair".
Peter Montellier, deputy editor of the Newcastle-based newspaper The Journal - which ran a high-profile campaign to save the bank, said: "Things could be a lot worse... 7,000 people could be out of work."
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