Children's Secretary Ed Balls will unleash an "expensive nightmare" with his planned replacement for discredited Sats, an expert warned today.

Mr Balls wants to introduce exams which children take when they are ready, instead of at the fixed age of 11.

Thousands of pupils in 360 primary schools are taking the new tests in reading, writing and maths in a pilot scheme across England this week.

But Tim Brighouse, from the Institute of Education, said pupils would face a battery of exams "year after year" under the proposal. Professor Brighouse, London's schools czar until last year, warned that this summer's marking fiasco showed the assessment system was "crumbling".

His remarks follow the Government's decision to abolish Sats for 14-year-olds in the wake of the shambles affecting the tests this year.

Results for more than a million pupils were delayed after a string of administrative blunders and computer glitches hit the marking process. In an interview with the Association of Teachers and Lecturers' magazine, Professor Brighouse said the proposed new "single level tests" would be no better.

Under the plan, teachers would enter pupils for a simple "pass/fail" test set at national curriculum Level 4, for example, when children are judged to be ready. When they are deemed to have moved up to Level 5, pupils will be entered for another test. Currently, pupils all take Sats on the same day in the last year of primary school and can be awarded a range of grades.

Professor Brighouse said: "People realise the system is creaking and crumbling. Look at the Sats this year. And with single level tests you could have kids taking them year after year. If they are all externally marked, you've got an expensive, bureaucratic nightmare."

He called for teachers to be accredited to run assessments in their own schools.Ofsted would then test a sample of pupils and compare the results. "Then it [Ofsted] could license the teacher for assessment for the next five years, say, and you would have a reliable, rigorous system. It would be easier to do and a damn sight cheaper."

ATL general secretary Mary Bousted said national testing of pupils had been "totally discredited".

"It is a mystery to me why the Government is considering multiplying the number of tests in primary schools.

"The system will not be able to cope, and it will be a joyless experience for teachers and pupils," she said.

The Department for Children, Schools and Families insisted the single level tests "remain a viable alternative" to Sats. They could be rolled out nationally from 2010 if the pilot is successful.

A DCSF spokeswoman added: "The evaluation will take into account the impact on workload, and costs."

In September, a separate report warned that the new assessments could prove unreliable, dominate lessons and harm pupils' education. Chris Whetton of the National Foundation for Education said the tests could trap pupils at a higher level than they are ready for.

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