Use of mainframe technologies, from suppliers such as ICL and IBM, fell from 42% in 2003 to 25% in 2004, according to the Socitm IT trends survey.
The main beneficiaries of the decline in the mainframe are Microsoft and Linux, the survey found. Nearly 60% of councils expected their use of Window XP to increase over the next year.
Linux is starting to plateau at 44% this year, compared to 42% last year.
John Serle, editor of Socitm's IT trends survey, said, "A lot of people that have adopted Linux think it is very cost effective. It will become part of the product mix to maintain pressure on other suppliers."