The U.K. bankers and regulators charged with reviewing Libor in the wake of regulatory probes are resisting calls to overhaul the rate because structural changes risk invalidating trillions of dollars of contracts.

The group, established by the British Bankers' Association in March after probes into allegations that traders rigged the London interbank offered rate, may propose a code of conduct for banks and impose greater scrutiny of Libor's correlation with other financial data over time, according to three people with knowledge of the discussions who asked not to be identified because the talks are private. It won't propose structural changes such as basing the rate on actual trades or taking away oversight of the benchmark from the BBA, the people said.

Libor is determined by a daily poll that asks banks to estimate how much it would cost them to borrow from each other for different timeframes and in different currencies. Because banks' submissions aren't based on real trades, academics and lawyers say they are open to manipulation by traders. At least a dozen firms are being probed by regulators worldwide for colluding to rig the rate, the benchmark for $350 trillion of securities.