Weight Watchers confirmed the split, citing changes in its creative strategy as the reason. However, the company refused to say whether it would seek a new agency.
The £7 million account was won by JWT Label, which specialises in female brands, in April last year after a pitch against Clemmow Hornby Inge and Delaney Lund Knox Warren & Partners.
The agency was briefed to create a campaign to encourage people to join their local Weight Watchers club, but failed to crack it.
However, JWT's sister direct marketing agency RMG Connect will continue to work on the company's DRTV and responsive press advertising business.
A joint statement read: "JWT Label and Weight Watchers today announced that after 15 months, they will no longer be working together, citing changes in creative strategy as the reason. RMG, JWT's sister agency responsible for direct, will continue to work on the account."
Mel Stubbing, Weight Watchers UK's senior vice-president, said: "It is an exciting and challenging time for the Weight Watchers brand and we will be continuing to develop and crystallise our plans for growth."