There has been a big jump in the number of primary schools in England setting optional tests for pupils.

The increase - 25% in a year - is such that there is a shortage of test papers.

Half way through the "tests month" of May, 1,600 schools still have still not received the papers - and might not get them for another fortnight, when the half-term holiday begins.

The sudden surge in demand is being attributed to the introduction of a new teachers' pay system, linking salary increases to pupils' performance.

Primary school pupils have to sit tests at the ages of seven and 11 - those for 11-year-olds being used for the performance tables published at the end of the year.

But similar "standardised assessment tests" have been made available since 1997 for eight, nine and 10-year-olds if schools choose to use them to measure progress.

Two reprints

The national curriculum tests in England are administered by the Qualifications and Curriculum Authority (QCA).

It says it has had orders from about 20,000 schools this year for the optional tests, up from 15,000 last year.

A spokesman said schools had been sent order forms last September, with a deadline of the end of November.

Those orders were met in full - but more came in at the start of the year.

A reprint was done and these were also met in full - but yet more orders came in during March and April, hence the backlog.

A second reprint has been done for the outstanding 1,600 orders. They should start going to schools next week.

Delay

The QCA expects them all to be delivered by Friday 26 May - the start of the half-term holiday. So some schools will not be able to carry out the tests until June.

There are fixed dates for the mandatory national curriculum tests - those for 11-year-olds start on Monday in England.

The optional tests can be taken at any time. But schools like to do them all at the same time to minimise the disruption to normal lessons.

Chris Davis, spokesman for the National Primary Heads Association, attributes the big rise in demand for the tests to the introduction of performance-related pay for teachers in England in September.

This requires teachers to be able to show "evidence that as a result of your teaching your pupils achieve well relative to their prior attainment..."

"In my school we were using the optional tests anyway," Mr Davis said.

"But I can't think of any other factor that's changed - apart from the tests having bedded down now so they're more reliable.

"We are still waiting for ours to be delivered - there's a national shortage."