Northern Rock has agreed to sell £2.2bn, or 2%, of its mortgage assets to US investment bank JP Morgan.
The troubled bank is up for sale
The troubled lender said it would use the funds to reduce the more than £25bn of emergency loans it has been given by the Bank of England.
Northern Rock said the price JP Morgan was paying represented a 2.25% premium over the value of the assets.
The lender first needed a loan from the Bank of England in September last year because of a global credit squeeze.
Looking for buyers
Northern Rock is up for sale. The two leading potential buyers are Richard Branson's Virgin Group and investment fund Olivant.
What is perhaps more important in a symbolic sense, the Rock does not appear to have received a fire-sale price
Some analysts say the credit squeeze may make it difficult and expensive for a private buyer to borrow the funds needed to fund the deal.
For this reason, nationalisation of the Rock is seen as a real possibility.
Northern Rock also said that its pension fund now has a £100m deficit. This would have to be taken on by any buyer.
It has also transferred funds from its final salary pension scheme from shares into more conservative investments, such as government bonds and cash deposits.
BBC Business Editor Robert Peston said that Northern Rock's mortgage sale "may not be huge progress, but at least the trend is in the right direction".
"And, what is perhaps more important in a symbolic sense, the Rock does not appear to have received a fire-sale price."
Our correspondent revealed that the Bank of England, and therefore the Treasury and taxpayers, have already incurred a notional loss on one element of its exposure to Northern Rock.
The so-called premium on the interest payable on the Rock's loans from the Bank of England is being rolled up into subordinated debt rather than being paid in cash.
This subordinated debt now has a market price of 65 pence in the pound, implying that the government is incurring a 35p loss on every pound of subordinated debt it receives from the Rock.