But in an interview with Sky News' Murnaghan programme, Sir Mervyn - who has just over a month left of his decade-long tenure as governor of the Bank of England - publicly questioned the policy.
"I'm sure that there is no place in the long run for a scheme of this kind," he said.
"This scheme is a little too close for comfort to a general scheme to guarantee mortgages. We had a very healthy mortgage market with competing lenders attracting borrowers before the crisis, and we need to get back to that healthy mortgage market.
"We do not want what the United States have, which is a government-guaranteed mortgage market - and they are desperately trying to find a way out of that position."
He went on: "So, we mustn't let this scheme turn into a permanent scheme... When is the right time to terminate it will depend on economic conditions at the time."
The BBC's business correspondent Joe Lynam said Sir Mervyn was concerned that, should the scheme become permanent, it could leave taxpayers exposed to billions of pounds in private mortgage debt for years to come.Interest-free loans
The outgoing governor's comments about Help to Buy echo those of the Treasury Select Committee, which warned in April that the government would come under "immense" pressure to extend the scheme in three years time.
Any such decision would be made by the bank's incoming governor, Mark Carney - the Canadian who was hand-picked by the chancellor to succeed Sir Mervyn.
Help to Buy consists of two elements - an "equity loan" scheme and the mortgage guarantee.
Under the equity loan, new or existing homeowners will need to raise a deposit of 5% of the value of the property they want to buy, but can borrow up to a further 20% from the government on an interest-free basis. The biggest loan available will be £120,000.
In total the government will be guaranteeing £12bn worth of lending.