U.S. lenders would have to offer mortgages with at least a 20 percent down payment if they want to repackage the loan to sell to other investors without keeping some of the risk on their books, according to a proposal regulators are considering on Tuesday.

The Federal Deposit Insurance Corp board is scheduled to vote Tuesday morning on the proposal that is intended to restore lending discipline and define the safest form of mortgages that can be sold to investors.

Last year's Dodd-Frank financial law requires firms that package loans into securities -- a practice known as securitization -- to keep at least 5 percent of the credit risk on their books.

The provision is meant to force securitizers to have skin in the game, so they don't churn out poorly underwritten loans and then pass along the risk to investors, as happened during the financial crisis.

Mortgages sold to Fannie Mae and Freddie Mac would be able to escape the risk retention requirement, at least while the mortgage finance giants remain controlled by the government.

(Reporting by Dave Clarke; Editing by Tim Dobbyn)