Canada Mortgage and Housing Corp. is putting a cap on the amount of mortgage-backed securities sold by banks that it is willing to guarantee.

A spokesperson with CMHC confirmed media reports Monday that the national housing agency will, effective immediately, limit banks and other mortgage lenders to $350 million worth of new mortgage-backed securities per month. The decision comes in the wake of "unexpected demand" for the guarantees, a spokeswoman for CMHC said in an emailed statement.

Under the National Housing Act Mortgage-Backed Securities (NHA MBS) program, banks have been able to securitize large portions of the mortgages they carry on their books. Because those securities are backed by CMHC, not the banks themselves, they're able to go out and lend that freed-up money to new homebuyers at lower prices, which adds fuel to Canada's housing fire.

Earlier this year, Ottawa announced it would limit the amount of those mortgage-backed securities that it would guarantee to $85 billion this year. That's a rise from from $76 billion in 2012.

But by the end of July, barely over halfway through the year, the banks had already tapped the program for as much as $66 billion, hence the need for the cap to stay under the annual limit.

CMHC said Monday no one lender will get guarantees for more than $350 million worth of securities per month, from now on.

The move takes some of the air out of the housing market by forcing banks and other lenders to be responsible for the risk of mortgage defaults, instead of being able to pass that risk on to government and taxpayers via the CMHC.

It's the latest move from a government that's getting increasingly vigilant about its attempts to cool down the housing market. After loosening rules to allow for no-money-down mortgages of more than 40 years a half-decade ago, the federal government has taken multiple steps to ratchet  those rules tighter again, limiting new mortgages to no longer than 25 years, and requiring a minimum down payment of five per cent of the value of the home.

Those moves were targeted directly at the homebuying public. But moves such as the one revealed Monday target the banks themselves, by effectively limiting the amount of money they have at their disposal to lend out in mortgages.

In the spring, Finance Minister Jim Flaherty went as far as publicly criticizing a number of lenders for encouraging reckless spending by offering mortgages with historically low interest rates.

The last time CMHC disclosed its data, the housing agency had $562.6 billion worth of mortgages on its books, getting close to its legally mandated limit of $600 billion.