After being rigged by some of the world’s biggest financial institutions, the London interbank offered rate, the benchmark for more than $300 trillion of securities and loans, is now increasingly being set by a smaller group of banks.

Bank of America Corp., Citigroup, Bank of Tokyo Mitsubishi UFJ Ltd., Royal Bank of Canada, Sumitomo Mitsui Financial Group Inc. and Lloyds Banking Group’s submissions have been used in setting the rate on an almost daily basis in the past four months as data compiled by Bloomberg show. Two years ago, none of the 18 designated lenders made it into every fixing of the measure, which excludes outliers by stripping out the four highest and lowest contributions.

The U.K.’s Financial Services Authority recommended Sept. 28 that LIBOR have a broader group of contributors, while acknowledging that developing an alternative would be too disruptive to borrowers around the world because the rate is so embedded in the financial system.

Traders have said for years that LIBOR was being rigged. Those suspicions were confirmed in June, when Barclays Plc (BARC), Britain’s second-biggest lender by assets, paid a record 290 million-pound ($468 million) fine for manipulating the benchmark. Bloomberg News has found that at least a dozen banks are being probed by regulators worldwide for allegedly colluding on LIBOR submissions to profit from bets on derivatives.