Ashurst and magic circle trio Clifford Chance (CC), Freshfields Bruckhaus Deringer and Linklaters have each been retained by the big four high street banks after the Competition and Markets Authority (CMA) launched its investigation into the current accounts and small business banking market last week.

The firms were originally instructed by the banks in July when the CMA first began a consultation process reviewing alleged lack of competition into the market with a view to launching a formal probe.

Ashurst has retained its role acting for Lloyds Banking Group with a team led by global head of competition and EU law Nigel Parr. The vote of confidence from the bank follows Ashurst's reappointment to Lloyd's core legal panel last month.

Freshfields' team acting for HSBC is being led by head of the firm's London antitrust, competition and trade group Rod Carlton and competition partner Andrea Gomes da Silva.

CC and Linklaters are understood to have maintained their spots as advisors for the other two main high street banks, Barclays and Royal Bank of Scotland (RBS) respectively.

CC M&A, corporate, antitrust and competition specialist Greg Olsen is acting for Barclays, while competition partner Simon Pritchard at Linklaters is taking the lead role for RBS alongside managing associate Ronan Flanagan.

Amongst the issues under investigation by the CMA are the lack of challenger banks to compete against the big four, a perceived dominance they have in terms of the small business lending and current account markets, as well as barriers to effectively comparing and switching between banks.

The investigation, which is likely to last 18 months, could see banks forced to sell off branches or roll back on free current accounts, both identified by the regulator as potential barriers to entry.