The National Association of Schoolmasters Union of Women Teachers has advised members to refuse to mark the optional maths and English test papers to prevent increases in workload.
Instead, teachers should insist that schools pay for them to be marked externally.
The union claims the tests will become mandatory once the key stage 3 literacy and numeracy strategies are introduced from September. They are already compulsory for the 205 secondary schools taking part in the KS3 pilot. Voluntary tests for eight, nine and 10-year-olds have become firmly established and are now used by about 95 per cent of primary schools.
The teacher unions are in talks with the Government on reducing workload and are pressing for a 35-hour week.
However, a report from the Paris-based Organisation for Economic Co-operation and Development this week reveals that this would result in English teachers having one of the shortest workng weeks in the developed world.
Mexico and Portugal are the only countries outside the UK where teachers are expected to work 35 hours or fewer. Teachers in Korea are expected to put in a minimum of 44 hours per week in and out of school, while their counterparts in Sweden and Germany complete a 40-hour week.
The report also says that English and Scottish teachers earn more than many of their overseas colleagues. In 1999, a primary teacher in England with 15 years' experience would have earned pound;22,410 compared to an OECD average of pound;18,391.
However, teachers in the UK pay for their higher salaries with larger classes. Teacher ratios in the UK were 22.5 at primary level and 17.4 at secondary compared to averages of 18 and 15.2 in other countries.
Nigel de Gruchy, general secretary of the NASUWT, said that while in most other countries teachers worked no more than a 40-hour week, independent research showed that teachers in England worked an average of 53 hours.
'Education at a Glance' is available on-line at www.SourceOECD.org