The UK's governance watchdog is investigating PricewaterhouseCoopers (PwC) over its auditing of Barclays Capital Securities on the investment bank's compliance with rules governing the protection of client money.

The Financial Reporting Council's Accountancy and Actuarial Discipline Board said on Friday its investigation would cover PwC's conduct in preparing reports on Barclays Capital, the investment banking arm of Barclays Bank, to the Financial Services Authority.

The reports related to Barclays Capital's compliance with client asset rules between 2001 and 2009, the AADB said.

In January, the FSA fined Barclays Capital £1.1 million for mingling client money held in sterling money market deposits with the its own funds for five to seven hours each trading day in that eight-year period. The amount of client money that was mixed grew to £387 million in 2009 from £6 million in 2002, the FSA said at the time.

"We will cooperate fully with the AADB investigation, and we will be defending our work vigorously," PwC said.

The FRC is also looking into Deloitte's auditing of the Royal Bank of Scotland.

Last year, the FRC asked Ernst & Young, another Big Four auditor, to explain its auditing of investment bank Lehman Brothers before it went bust in 2008.

The European Commission is currently looking at whether to force the Big Four auditors -- KPMG, PwC, Deloitte and Ernst & Young - to break up and rename their businesses in a move aimed at eliminating conflicts of interest and audit shortcomings highlighted by the financial crisis.