Teacher assessment is not strong enough on its own to measure pupil ability, according to a union leader backing a boycott of school Sats.
Large numbers of primary teachers will continue to use standardised science tests this year even though Sats for this part of the curriculum finished last year, said Mick Brookes, general secretary of the National Association of Head Teachers.
Teachers will need test results in order to reach accurate assessments of their pupils' abilities, he warned.
"On its own, teacher assessment is not strong enough yet, but neither are tests," he said. "A combination of both should get a better result. It's highly likely that schools will want to use standardised tests to underpin their opinions."
The NAHT is leading a joint campaign with the NUT to boycott this year's primary school Sats in English and maths.
The unions want to replace the tests with teacher assessment and stop the collation of results in high-stakes league tables. But Mr Brookes stressed that this does not mean the union is against testing.
"People don't have a problem with the tests but with their administration and league tables," he said.
"If schools want to use their own assessment systems, that would be fine, and if teachers find it useful to use standardised tests as part of the accreditation of their assessment, that's up to them."
The expectation that primary schools will continue to use standardised tests is based on the response when key stage 3 tests were scrapped, but secondaries continued to use them.
"Our Year 6 teachers are thinking of using a past Sats paper, but they will also use observations of practical experiments to level science," said Anita Gorringer, deputy head of Comber Grove primary in Southwark, London.
But a new method for judging pupils' ability in science - Assessing Pupil Progress (APP) - is being piloted.
In this system, teachers compare pupil performance against a range of indicators. Materials to support teachers using APP were published last month, but national training is not due until the summer.
Early feedback from the pilot suggests that APP is encouraging schools to be more adventurous with science lessons.
Liz Lawrence, chair of the primary committee of the Association for Science Education, said anecdotal evidence pointed to schools modifying their teaching to achieve a more balanced approach to the subject.
The changes to the way primary science is assessed were made following recommendations from the Expert Group on Assessment, which said that the tests were not effective in assessing children's understanding.
This followed an Ofsted report which said tests had prompted school to cut back on practical science lessons in favour of written work.
APP is not a universally popular change, however, and there are concerns raised that it will prove to be too bureaucratic.