The Competition and Markets Authority (CMA) is investigating the market for personal current accounts and small business banking services and will decide what measures must be taken by May 2016.
Its chief executive Alex Chisholm said in a submission to New City Agenda, an independent political think tank, that the CMA could order banks to sell assets or tell them to provide clearer pricing and comparisons between different products.
Unlike previous investigations into the industry, the CMA has the power to order banks to take actions rather than just make recommendations, Mr Chisholm said, and those actions may also require changes to current laws and regulations.
Mr Chisolm said moves to stimulate competition, including ensuring customers can switch banks in seven working days, had not had the desired effect.
“They don’t seem to us yet to have had the transformative effect hoped for, and the long-standing concerns about competition in retail banking largely remain,” he said.
Britain’s biggest four banks - Lloyds, Royal Bank of Scotland, Barclays and HSBC - control more than three quarters of current accounts and provide nine out of ten business loans.
Some MPs blame a lack of choice for scandals that have engulfed the industry in recent years including the mis-selling of loan insurance and complex hedging products and shoddy treatment of small businesses in financial difficulty.
State-backed RBS and Lloyds could come under particular scrutiny in the investigation. Lloyds is Britain’s biggest provider of personal current accounts, while RBS provides services to more UK small businesses than any rivals.
Lloyds boss Antonio Horta-Osorio called on Tuesday for greater competition in banking services to small firms.
In a further submission to the think tank, Tesco Bank’s chief executive Benny Higgins criticised the way some of Britain’s biggest banks treated customers.
“Many of the banks get away with offering their customers a very poor deal - generating billions of pounds in revenue for themselves as a result,” he said.
Mr Higgins, a former RBS executive, said greater transparency in pricing was needed to stimulate competition.
He said: “I think that we can aspire to be a significant player but I think the nature of the market means it will take time.”
In another submission, Paul Pester, chief executive of new bank TSB said greater competition was “the only way to deliver the kind of banking customers want”.