Sinclair will become 100 per cent owner of the agency after agreeing to acquire Bruce's shareholding.
Sinclair will take a more hands-on role in the interim before a decision is made on who will become MD.
Bruce told PRWeek: ‘I have very different ideas to Tim on the future of the business and now seems the right time to look at other opportunities – I think I’ll do better on my own.’
He has worked both on the agency and in house sides of PR with roles as director of strategy and comms for Labour MP Alan Johnson's deputy leadership bid and for shadow education secretary Andy Burnham's bid to be elected as leader of the party.
Bruce also stated that there ‘has been a lot of change’ in the past few months, including client turnover and also the loss of deputy MD Mark Hanson who died at the age of 36 in March after battling against depression.
Bruce will leave in the next two weeks after finalising arrangements with the firm.
He added: ‘I will be keeping busy with some strategic communications consultancy work and I’ll be looking out for new opportunities. I’ll also be considering some non-executive director positions with some exciting start-ups.’