England's exams regulator this week raised the prospect of an end to national testing for 11 and 14-year-olds.
Ken Boston, chief executive of the Qualifications and Curriculum Authority, appears to be on a collision course with ministers after making the suggestion for the second time in a year.
It was immediately rejected by the Government which said there was "absolutely no question of moving away from externally marked tests".
Dr Boston suggested that the tests could, in the long run, be replaced by standardised teacher assessment if this could be shown to be as reliable and rigorous as national tests.
His comments came at the launch of the Institute of Educational Assessors in London this week.
Under Dr Boston's proposals, teachers would have access to a set of national tests, which they could set their pupils at any time during a specified period. Teachers would mark the tests and pass the pupils' work and scores to independent assessors for moderation. If such a system could be demonstrated as a success, the QCA might be able to recommend it to ministers as a "better process" than the current national tests.
Dr Boston said he was not in favour of teachers setting their own tests, because of "the impossibility of being able to strike a common standard nationally across all the classrooms in this country".
The system proposed bears striking similarities to that used since 2004 at key stage 1, where teachers are sent standardised tests and choose when to give them to their pupils.
In April last year, Dr Boston suggested that teacher assessment could replace testing within 10 years. But he received a similar brush-off from the Department for Education and Skills.
Dr Boston's suggestion received a caution from Mick Brookes, general secretary of the National Association of Head Teachers, whose annual conference last week voted to scrap national test league tables.
He said: "We would like to talk to Ken Boston and the QCA for their thoughts on this issue."
A DfES spokesman said: "Externally marked national tests provide the most reliable, objective and consistent measure of what young people have achieved.
"They are also vital for public accountability."