Ministers have re-stated their pledge that league tables will be superseded by school report cards - in a letter sent to all primary heads on Wednesday, days before they vote on whether to disrupt this summer's Sats.
The last-ditch attempt to sway wavering voters before the ballot starts on Monday also included guidance entitled "Getting the balance right", which advises heads not to drill children for the tests.
Members of the NUT and National Association of Head Teachers (NAHT) will have until the middle of next month to vote on whether to support a boycott.
The letter, from Schools Secretary Ed Balls and his minister Vernon Coaker, is "an update on how we work together to improve our assessment and accountability system".
It states that from 2011, the school report card will be introduced and "supersede" the current tables. The cards were announced in October 2008 and are being piloted in 300 schools.
The letter adds that the changes recommended by the Government- commissioned expert group on assessment, which included dropping the key stage 2 science test, are being implemented in full. However, it adds that the group recommended that until teacher assessment is more robust the KS2 tests in English and maths should remain. "We believe, therefore, that it is essential that the tests take place as planned," the letter adds.
But Mick Brookes, general secretary of the NAHT, dismissed the letter. "We want to see the end of the current system of league tables. We want to see them dropped this year," he said. "I think this letter is vague.
"We need a project plan with dates. I'm not surprised the DCSF sent it out to put its point of view across at this particular juncture, but it makes no difference to us, we want to see change in 2010."
Huw Thomas, head of Emmaus Catholic and CofE Primary in Sheffield, agreed. "It is no good just sending a letter, they have to give us their trust," he said.
"I will be voting no because I think it is too late in the year to boycott the tests. But I am not 100 per cent convinced with the school report card.
"And for Ed Balls to tell me to `get the balance right' when they are the ones who have unbalanced the system in the first place. They're the ones who need to get the balance right, not us."
The NAHT and NUT will send ballot papers out on Monday, with a closing date of April 16. The proposal is for action which frustrates the administration of the tests. Sats are due to take place during the week beginning May 10.
Original paper headline: Last-ditch bid to woo primary heads as Sats ballot looms