WASHINGTON (Reuters) - A U.S. regulator is filing lawsuits against major banks, accusing them of bundling subprime home loans into bonds that never should have been sold to investors, and causing mortgage giants Fannie Mae and Freddie Mac to lose billions, a source said.

The Federal Housing Finance Agency, which oversees Fannie Mae and Freddie Mac, is suing major banks soon, in a move that will deepen the mortgage litigation morass for banks.

The biggest banks already face the potential for tens of billions of dollars of payouts to settle regulatory charges of abusive mortgage lending and foreclosure practices, and other investor lawsuits over mortgage debt losses.

These payouts would reduce earnings and weaken capital levels, perhaps harming the ability of banks to lend money and provide much-needed life to a stalled housing market and weakened economy.

Fannie Mae and Freddie Mac have also been stricken by bad mortgages, and are under government conservatorship after having previously been quasi-private. The two firms guarantee bonds backed by mortgages, and are a crucial pillar for U.S. housing finance system.

News of the lawsuits was first reported Thursday night on the website of The New York Times.

The source declined to name specific banks, but The New York Times reported that the banks to be sued include Bank of America Corp, JPMorgan Chase & Co and Goldman Sachs Group Inc among others. Representatives of Goldman, Bank of America and JPMorgan declined to comment.

(Reporting by Margaret Chadbourn, additional reporting by Jonathan Stempel; Editing by Dan Wilchins and Matthew Lewis)