In total, five senior figures have quit - four of whom were on the board. The departures have forced Lewis to promote three new faces to the board.
The most recent staff to resign were creative director Jon Silk and director of media strategy Will Sturgeon - both former journalists.
Sturgeon will be moving to Harvard PR to take on the role of director of strategy and planning, while Silk will take on the role of senior digital consultant at Waggener Edstrom.
In addition, Lewis' sister agency Octane PR saw the departure of MD Kelly Hall back in July. Weeks later, general manager for London Anna Longstaff left to join b2b marketing agency The Marketing Practice.
All four staff members were on the ten-strong UK operating board for the agency.
Additionally, financial director Matt Long left last Friday.
Industry sources pointed to problems stemming from the departure of Lewis UK general manager Cath Pooley last year, with one source saying this led to 'difficult internal politics'.
Insiders also said the agency had suffered from key client BT reducing fees following a recent repitch.
A spokesman for Lewis said: 'BT budgets have decreased but we are still happy to be working with it. We are about to announce an 11 per cent growth in the agency and have expanded our offices globally to a total of 40 worldwide.'
The spokesman would not reveal who was being promoted to the board.
Lewis PR was set up in 1995 by Chris Lewis, a former managing director at Associated Newspapers.