He said: "The credit crunch is having a severe impact on all industry sectors and we are operating in a fiercely competitive environment.
"In order to maintain shareholder support for our very large growth investment programme - and to look after the interests of our customers - we need to try and maintain acceptable returns."
The company will therefore freeze pay for many of its 12,500 staff.
The firm recently released its sixth full corporate responsibility report, which explains how the firm is delivering affordable, reliable energy to its customers despite the economic downturn.
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