LONDON, Nov 9 (Reuters) - After Martin Sorrell's advertising group delivered more bad news about the sector on Thursday, he re-ignited a war of words with the man who succeeded him at WPP, Mark Read, who is also battling to win clients' dollars.

Read took over at WPP in 2018 after Sorrell was ousted in acrimonious circumstances from the firm he had built into the biggest ad group in the world over more than three decades.

WPP reported a worse-than-expected 0.6% fall on its top line in the third quarter as clients, particularly in tech, cut spending. Sorrell's S4 Capital also missed forecasts with a 10% decline in third-quarter like-for-like revenue on Thursday.

When asked by "Campaign" last month if he had been too slow to integrate back office functions across WPP's agencies, Read answered: "I don't think we've been slow, but we're dealing with 30 years of inactivity on that topic."

Sorrell, who still owns a small stake in WPP, said all the responses in his mailbag had been saying that was "a pretty pathetic excuse for his non-performance".

"The question is, when is he going to take responsibility?" he said in an interview on Thursday. "He's been at it for five years, so when is he going to take responsibility?"

Read is merging Wunderman Thompson and VMLY&R to create the industry's largest creative agency, and is streamlining back-office functions across media buying agency GroupM.

Sorrell said Read's problems included low growth, weaker margins than competitors, and a bribery case in China.

"Mergers don't paper over people issues," he said.

"There's a lot of issues he has to deal with and I think he lashed out in frustration."

A spokesperson for WPP said: "(Read's) comments were simply a response to a journalist's question about the integration of back-office systems." (Reporting by Paul Sandle; Editing by Jan Harvey)