One society offered a 24-year loan to a borrower who could apparently afford repayments on a 13-year mortgage, potentially earning the lender an extra £34,566 in interest.
The claims were made to Financial Adviser newspaper by mortgage brokers, some of whom wanted to remain anonymous for fear of being excluded from the panels of advisers used by lenders.
Under the MMR, lenders and advisers are now obliged by the Financial Conduct Authority to go through applicants' finances in forensic detail, to ensure they can afford to borrow even if mortgage rates rise.
One broker said it was "galling for the client to see five or six years being added to the deal, particularly when their existing mortgage payments are higher than the new payments being proposed".
He said the mortgage market review, which took effect last month, was prompting societies such as Nationwide to approve deals over a longer term, adding: "If the clients are not proactive enough to make overpayments, this practice is going to cost them."
The Building Societies Association said borrowers had to meet the required affordability checks, adding: "There are a range of products available on the market, including the ability to overpay if the borrower chooses."
Nationwide, headed by Graham Beale, reported record profits this week. It said independent surveys and awards showed the society remained "true to its mutual heritage by offering competitive products with long-term member value."