The NUJ has described new contracts offered to freelance photographers by Emap as "extremely intimidating" and urged them to negotiate with the publisher for a better deal.
The warning came as Emap Automotive and Active offered photographers a range of fresh contracts to address future copyright issues.
A full contract, covering work commissioned by Emap, grants it "the entire copyright throughout the world".
That means no extra payments for the re-use of photographs in Emap publications internationally. A limited contract, for work initially submitted by the freelance, grants Emap "exclusive, irrevocable and perpetual license to re-use the work".
There will be no extra payments for reuse in the commissioning title and a payment of between 5 and 10 per cent for use in Emap's international editions.
Under both contracts, syndication fees are be split 50/50 if the work is sold to a third party.
If a commission is cancelled before work is supplied to Emap, no payment will be made at all.
Emap confirmed there was a third type of contract — a single use option, whereby the publisher returns rights and pictures after one use. If agreed, Emap can re-use for additional fees.
John Toner, the NUJ's national freelance organiser, said: "The NUJ's take is that the contracts are extremely intimidating, but photographers should not be intimidated. I have advised, and will continue to advise photographers to try to negotiate better terms."
He added that the syndication fee was the industry standard, the re-use fee in Emap international editions was "too low" and that the freelance should be paid for work completed at the point of cancellation.
Emap has sent out contracts accompanied by letters from the respective editors, advising contributors to get in touch if they had any queries.
A spokeswoman for the publisher said the contracts were introduced to protect both Emap and freelances. She said: "No freelance is made to sign a contract they are not happy with.
"The only contract that gives Emap ownership is the full contract, and this is only put in place on work that Emap commissions — it is not issued on work that freelances send in. We would not expect to take the copyright from these images sent in to Emap."
David Hoffman, a spokesman for Editorial Photographers UK, said the contracts extend the publisher's licence or aim to own copyright without a proportional increase in the fee paid to the photographer.
"The fees are apparently the same as before, but the value of what Emap is asking for has been considerably increased and the photographer's ability to exploit their own work reduced," he added.
"Like any business, Emap is looking to buy low and sell high, but here it is using its huge market power to force photographers to sign away a significant part of their future resales income. Given the pressure on individual freelances these days, many will be too frightened of losing work to resist."
The move has echoes of a previous spat between photographers and IPC in 2002 when the UK's largest magazine publisher announced plans to reduce the number of contracts on offer to five, the most controversial of which was the "all rights" version.