The magazine was also criticised for taking an "unacceptable amount of time" in dealing with the complaint.
An article in Woman from 7 April last year quoted a source saying Cassidy, who played Sonia Fowler in the BBC soap, was "preparing" for her home workout weight loss DVD by visiting London's Esporta gym five times a week.
Cassidy denied the claim, saying she had lost weight solely through exercises from her DVD.
The magazine said both claims could be true – as using machines is not mandatory in gyms – and also ran a follow-up in which it offered a right of reply.
But the PCC ruled in favour of Cassidy. In its adjudication, published today, the watchdog said: "The commission has previously made clear that, while it does not expect publications to identify confidential sources of information, they should either supply on-the-record corroboration or offer an opportunity to reply when the accuracy of an anonymous source is questioned.
"The magazine had done neither in this case, resulting in a breach of clause one [the accuracy clause in the editors' code of practice].
"It was also regrettable that the magazine had taken seven weeks to respond to the commission during the investigation into this complaint.
"This was an unacceptable amount of time to deal with this relatively straightforward matter."
Woman's circulation in the second half of 2008, announced by ABC last week, was 347,585, down 6.4 per cent year on year.