The Treasury said Wednesday that eight major banks had thrown their support behind the plan, the Daily Mail reported. However, Nationwide -- one of the banks named by the Treasury -- said Thursday it was "happy to participate in exploring any potential solutions."

"At this stage, we are unable to comment on a particular scheme," a Nationwide spokesman said.

Barclays, the Royal Bank of Scotland and Northern Rock (OTCPK:NHRKF) also refused to give the deal a blanket endorsement.

The Treasury then said the banks had pledged "to develop the scheme."

Housing Minister Margaret Beckett refused to say how many homeowners the plan would keep from losing their homes. Reports Wednesday said she had suggested the plan would rescue only 9,000 homeowners.

Brown said his plan, which would guarantee banks against default, could help 10 million middle-class homeowners.

Estimates of the plan's cost vary widely. The Treasury estimated a $150 million price tag, while others believe the cost could reach $1.5 billion.