Beattie McGuiness Bungay (BMB) was the majority investor in Seven Dials at its launch in May last year with a stake understood to be 90 per cent and Kelner holding the remainder.
PRWeek has learned that Kelner bought the agency outright in December for an undisclosed sum and has moved it out of BMB's offices.
Kelner said: "We grew so quickly that we made a decision to buy the business out of BMB and we are now fully independent."
BMB was unavailable for comment on the split, which appears to have occurred around the time it emerged in December that one of its three founders, Andrew McGuinness was leaving.
McGuinness was also chairman of Seven Dials and according to Kelner is working two days per week at the agency.
Seven Dials is now based in new offices in Poland Street, Soho. Kelner acknowledged that the agency "couldn't have got to where we are today without BMB", but added "we are enjoying being the masters of our own destiny".
The only obvious record of Seven Dials' ownership on Companies House shows a company called Seven Dials Communications Limited was incorporated – with Kelner as the sole shareholder – in November 2013, six months after Kelner and BMB went public about the launch.
BMB has yet to file its accounts for 2013, which could shed more light on the transaction.
At the time of the launch McGuinness and Kelner talked up the benefits of links between the two agencies.
McGuinness said of BMB's decision to venture into PR: "Most clients aren’t looking for an all-in-one proposal but, if you offer them a credible way of accessing a best-in-class resource, many will do that."
Kelner said: "In a sense, we have the best of both worlds. We’ve got autonomy as a PR agency, but the ability to draw on BMB’s creative strengths and the infrastructure that they’ve built up."
The agency's founding clients were upmarket retailer Fortnum & Mason, investment firm Aberdeen Asset Management, and online drinks retailer 31Dover. It subsequently picked up Global Radio and sushi chain Itsu.
The agency has around seven staff including new business director Eleanor Conroy, a former director at Freuds.
McGuinness and fellow BMB founder Trevor Beattie previously worked with Kelner when their agency held The Independent’s ad account.