The move has angered press directors, who are used to negotiating terms with Emap on all its titles. One agency refused to continue discussions and threw Emap's ad team out of its offices.
Emap is understood to have singled out Grazia for the fixed increase, which applies to agencies' established individual terms as well as the magazine's rate card. It has not proposed such strict changes to other strong-performing titles, such as Heat and Closer.
Press buyers said Emap was unusually firm about the increase, and that the publisher claimed it would rather turn away business than negotiate lower rates.
The magazine posted an ABC increase of 23.1% in 2006, to 210,200, with sales up 20% in the second half of the year.
It now sells more copies per week than monthly magazines Elle, Easy Living and InStyle.
Heat's circulation was up 4.1% year on year to 598,623 and Closer's rose 6.2% to 614,141.
Emap is understood to consider Grazia's ad space at a premium due to the pagination limits and time restraints of its weekly frequency.
But agencies criticised the move, which came at a peak time for ads with London Fashion Weekend, an event sponsored by Grazia.
One, who described Grazia as a "fantastic proposition" that fashion clients saw as essential, said Emap's announcement "went down like a lead balloon".
He added: "I've never come across such high rate increases. When the magazine market is so volatile and Grazia is succeeding while others perform less well, it took me by surprise that they would come down so hard on price."
Abby Corvosso, publisher of Grazia, said Emap had been consistently transparent since the magazine's launch about its premium positioning, which was reflected in its ad rates.
She added: "It's a difficult audience to buy for, and, as a weekly, it's important that advertising doesn't restrict frequency of purchase.
"We want to leave people wanting more, which makes the ad proposition very different from a monthly."
This article was first published on Media Week