Research by the National Association of Head Teachers, also found that the strategy's introduction lowered morale in more than half of the schools surveyed.
The NAHT spoke to 1,261 schools, of which the overwhelming majority, 95 per cent, said they were using the national literacy strategy - which includes the literacy hour - introduced a year ago. In 98 per cent of cases the strategy was said to have increased working hours, with 90 per cent of schools reporting a large or very large rise. Just over half said it had had a negative effect on morale when introduced.
The most common reaction was to make room for the strategy by cutting back the time available for the non-core subjects on the national curriculum. Design and technology suffered most at both key stages 1 and 2, but history, physical education, art, music and geography also found their time reduced.
Nearly nine out of 10 schools at key stage 2 (ages 8 to 11) cut back on those subjects, allowed under a temporary relaxation of the curriculum.
An improvement in results was also detected in a second, smaller-scale study by academics at Newcastle University. But authors of this survey, based on 39 local schools, warned that the literacy hour had thrown up a "polarisation effect", with some primary teachers witnessing a growing gap between average and above-average children, and those with special educational needs.
Dr Frank Hardman, senior lecturer in education at the university, said his department would now track individual children on a longer-term basis to work out the true effectiveness of the literacy hour.
The 39 schools in the Newcastle survey agreed overwhelmingly that the strategy would support effective literacy management by providing continuity, monitoring and evaluation. However, they were more divided over whether it would raise standards for every pupil. Researchers found general concern over the lack of creativity in writing.
NAHT general secretary David Hart said the new literacy strategy had developed teacher skills and knowledge and had a positive effect on pupils' reading skills. But he said teachers would "prefer to use their professional judgement to implement the strategy in a way that would best meet the needs of individual schools".