The pair - whose departure from the end of December was announced last week - sold Tag in July 2011 to Williams Lea, which refused to comment on Tag's new leadership structure.

Rumours of possible replacements have been flying around adland; however, as yet no concrete information has surfaced as to who will replace Parish, who as chief executive became synonymous with the Tag brand.

One industry source said: "In a lot of ways Steve and Rich were Tag - if you take those two out then what have you got? It's a classic problem you get when you buy a business like that: how do you maintain that brand once the drivers behind it have left?

"It's not clear yet what strategy Williams Lea has got for Tag [after Parish leaves] but maybe they've looked at it and decided by the nature of what the business does that the clients are quite sticky."

Mike Newman, founder of Intelligent Outsource, said that it was not clear who within Williams Lea had direct responsibility for Tag and argued that it would be difficult to replace Parish with one of his subordinates.

"Unless something has radically changed in the last three years, there isn’t a succession structure in place," he added. "Steve’s exit has left them with a major leadership issue. They’ve got some excellent people, but I honestly think they will have to parachute someone in because of the way the business has been run."

Williams Lea has declined to comment on the perceived leadership vacuum at the top of Tag other than to say that it would be brought together with its Marketing Solutions business "to offer clients a wider range of services to improve their marketing efficiency".

Another source said: "The challenge they face is to move from a very successful but autocratic business into one that is run more like Williams Lea. They bought Tag as an organisation but they were also buying Steve as its head and taking the head off a business like that is always a risk.

"I honestly think this is the step change they probably needed to make because now they will find out what they really bought, but they’ve been left with a vacuum that they’re going to need to fill very quickly – I would expect there to be another announcement soon on who will be heading up Tag."

Parish, who was not available for comment, was said to be leaving to pursue "other interests outside of the business", although sources said they did not think it likely that he would take up a permanent role without an equity stake.

"I think he’s more likely to consult or to find another business to invest in," one source commented.

"The pair of them have got the insight and they've got the money so potentially they might look to do something similar but different," another added. "Then again, they might just say, 'you know what - we've done that' and do something else altogether."