The nation’s largest banks are facing a fresh torrent of lawsuits asserting that they sold shoddy mortgage securities that imploded during the financial crisis, potentially adding significantly to the tens of billions of dollars the banks have already paid to settle other cases.

Regulators, prosecutors, investors and insurers have filed dozens of new claims against Bank of America, JPMorgan Chase, Wells Fargo, Citigroup and others, related to more than $1 trillion worth of securities backed by residential mortgages.

Estimates of potential costs from these cases vary widely, but some in the banking industry fear they could reach $300 billion if the institutions lose all of the litigation. Depending on the final price tag, the costs could lower profits and slow the economic recovery by weakening the banks’ ability to lend just as the housing market is showing signs of life.

The banks are battling on three fronts: with prosecutors who accuse them of fraud, with regulators who claim that they duped investors into buying bad mortgage securities, and with investors seeking to force them to buy back the soured loans.