Northern Rock says arrears rates are increasing
Ministers have been urged to act after it emerged that Northern Rock is repossessing 50% more properties than the industry average.
Shelter said ministers had a "moral duty" to help people stay in their homes while the Lib Dems said the figures were "difficult to swallow".
Gordon Brown pledged more help but said ministers did not run the business.
By the end of September, the state-owned lender had 4,201 seized homes, up from 2,215 at the end of last year.
Northern Rock rejected suggestions that its approach to repossessions was "overly aggressive", saying the measure was only ever used as a "last resort".
But charities have expressed anger that after being bailed out by the government last year, Northern Rock's repossession rate in the first half of 2008 was double that of the industry as a whole.
Shelter said repossessions should only occur if "absolute necessary" and that people should be given every assistance, including free independent advice, to help them stay in their homes.
"It seems a bit perverse that ministers who a few months ago were lecturing lenders about their responsibility towards homeowners in arrears are now allowing companies that are state owned to repossess people's homes so aggressively," said its chief executive Adam Sampson.
"We realise the government cannot avoid all repossessions but it must ensure a dignified and planned exit from mortgages that are held by the newly nationalised banks and hopefully try to allow people to stay in their homes wherever possible."
In July, Northern Rock said the number of borrowers three months or more in arrears on their mortgage payments had doubled in the first half of the year as economic conditions worsened.
Experts said Northern Rock's repossession rate was likely to be higher than average as it offered more 100% and higher loans than most lenders and since many of its less vulnerable customers remortgaged elsewhere after the firm was nationalised.
The government announced a £300m package of measures in September to help people stave off the threat of repossessions.
Help is being offered for people on benefits to pay interest on their mortgages while people will be able to sell all or a share in their property to a housing association or other social landlord, enabling them to remain in their property.
Mr Brown acknowledged that more homeowners were finding it hard to meet their payments and said the government was looking at further ways to help borrowers through the benefits system.
But he deflected criticism of Northern Rock itself, saying it was "important to note that it is at arm's length from government".
"It is not a company we are running on an everyday basis," he added.
The Lib Dems said lenders should be under a legal obligation not to issue repossession orders until they had explored every means of helping people, including renegotiating their loans.
"People get their household finances in a mess and they can be straightened out through quite simple measures like deferring a payment here or cutting some costs there," said leader Nick Clegg.
"That is probably the best way to make sure we do not simply move to this default position of mass repossessions up and down the country."
Leading left wing Labour backbencher John McDonnell also urged government action on repossessions.
He said: "We fully nationalised Northern Rock, yet the government's bank is becoming the most ruthless repossessor under the cosh of government pressure to repay the loans. The Government is in danger of being seen as protecting banks while ignoring people.
"The government needs to come up fast with a 'recession-proof' strategy of halting repossessions and converting mortgages into homes for social rent."