In an interview with The Drum, Sorrell said the company quickly realised it had to address the biggest problems first and sort through them "line by line". It took WPP between seven and 10 working days to sort it out.

"It lasted intensely for about a week. We had heavy internal communications that we handled reasonably well and we learned that it was important to not try and do everything. It didn’t affect the whole company. But we moved pretty quickly to contain what we could contain."

He said that the company has held a summit since the attack, with about 50 chief executives, to discuss what happened. "Tactically you have to be very sure that you have gone through the fire drill," he said.

WPP is investing an incremental £10-£15m a year on measures to prevent another attack. He also praised his staff for how they responded in the crisis and cited the help its partner IBM had given. 

"They were very resourceful. People in high pressure situations responded magnificently."