The survey of 256 of the 4,700 schools taking part in the pilot found one in 10 spent more than 30 hours outside normal classroom time on test-related paperwork. However, more than half (54 per cent) felt they had a similar workload to previous years.
The pilot offers schools the option of escaping the formal 2004 tests and instead giving children last year's tests at any time in the year, as part of teacher assessment. Pupils do not lose out by being tested earlier than their peers because pilot schools only report the teacher assessments and not test results.
Chris Keates, acting general secretary of NASUWT, said: "It is evident from the survey that the impact on teachers' workload has varied according to the implementation strategies used by schools and local authorities. It would be unacceptable for the system to be rolled out nationally without such inconsistencies being addressed."
But, she said, teacher led assessment of seven-year-olds had been "generally welcomed".
The Leeds university team evaluating the project found that most schools given the chance to do the 2003 tests had opted to stick to the 2004 tests and timetable, as The TES revealed last week. A decision on whether to extend the pilot to all schools in 2005 will be made in September.
Ms Keates said: "While more than half of respondents said the new arrangements did not compare badly to previous ones, in terms of workload and bureaucracy, a third of teachers believed they did. This level of dissatisfaction must not be overlooked."
Wales has already scrapped key stage 1 tests. The Welsh Assembly announced the KS2 tests will also be replaced by teacher assessments from next year.