Freak storms are being blamed for the biggest marking error in the 80-year history of America’s Sat tests.

Some 4,000 students heard earlier this month that the marks they thought they achieved in the college entrance exam were wrong.

Unusually humid conditions and torrential downpours in the north-east of the country on the day of the test (October 8 last year) caused the answer sheets to swell with moisture. In some cases, they were misread by machine readers, said a spokesman for Pearson Educational Measurement, the company that marks the US equivalent to A-levels.

But the bizarre explanation belies a serious situation. Universities may have to reconsider decisions on admissions made on the basis of erroneous scores, said Brian O’Reilly, executive director of Sat Information Services at the College Board, a New York non-profit organisation which administers the tests and outsources the marking operation to Pearson, based in Iowa.

Internal checks failed to identify the glitch, which only emerged after two students challenged their scores. This prompted Pearson to undertake an internal investigation and re-scan all 495,000 answer sheets from the October test .

“If they hadn’t come forward, this error might never have been discovered,”

said Mr O’Reilly.

“There have been times when individual students’ exam sheets have been mis-scored because they’ve become mangled or something like that, but never a technical error that has affected as many as 4,000.”

All the affected students’ tests were underscored. In 95 per cent of cases, the discrepancy was fewer than 100 points on the 2,400-point tests - not enough to sway college admissions decisions, said Mr O’Reilly.

But 150 students’ scores were more than 200 points out, with one nearly 400 points awry.

The Pearson spokesman said the firm had since added new technology to “check for paper expansion”.