Ofsted will not penalise schools that took part in last week's Sats boycott, as long as they can present inspectors with "robust data" on pupil progress and achievement, it has emerged.
Heads' union the NAHT said the inspectorate has assured its officers that schools are safe from being marked down, as long as they have reliable performance data based on methods such as internal testing and teacher assessment.
Ofsted itself, which has in public remained cagey about how it will treat boycott schools, this week released initial guidance for schools being inspected late this term.
Inspectors who would not have access to provisional Sats results should "exercise professional judgment in weighing all the evidence gathered, setting this against the credibility and accuracy or the school's teacher assessment data", it said.
Mick Brookes, general secretary of the NAHT, said: "We have spoken with Ofsted and it has told us what it expects is `robust data' on children's progress. It has been extremely helpful on this and we are certain schools won't suffer. We would ask any head who does experience a problem to report it to Ofsted's central office."
The real impact of the boycott on inspections will not became fully apparent until September, when inspectors would usually want to see full 2010 Sats results.
The news comes a week after thousands of primaries in England boycotted the Sats. Many schools decided to sit past papers, mark them internally or even exchange papers between schools for moderation.
Surveys revealed that participation was extremely varied in different parts of the country, and some heads have complained of local authorities using "bullying tactics" to make schools back out.
The NAHT says it did not survey members to find out how many took part and teaching union the NUT could only say the figures it had gathered were "very encouraging".
A TES survey of half the local authorities in England, expanded nationwide, indicated that at least 4,000 schools, or a quarter of the total eligible, had taken part.
The Qualifications and Curriculum Development Agency was this week collating data on the number of papers that had been returned for marking, although it said some papers may have been returned unused.
The final numbers will be of interest to the new Education Secretary, Michael Gove, who is expected to meet soon with Mr Brookes and Christine Blower, general secretary of the NUT.
Mr Brookes said all indications prior to the election were that Mr Gove would welcome sit-down talks on the future of assessment. Previously, Mr Gove has said he would like to reform the Sats, possibly moving them to Year 7.