Damian Green, Conservative education spokesman, announced this week that the number of children who skipped school had increased overall by 15 per cent since 1996.

He added that the number of secondary pupils who played truant at least once had risen by 25 per cent, despite Government spending of pound;650 million on anti-truancy initiatives.

Figures provided in a written parliamentary answer show that the recorded number of secondary pupils who play truant has increased from 423,000 in 19967 to 566,000 in 2002.

But researchers point out that the statistics for 1996 were unusual because 27 local authorities did not report their results that year.

John Howson, visiting professor at Oxford Brookes University and director of Education Data Surveys, said that several of the LEAs had not provided figures because of the changes caused by the introduction of unitary authorities.

“If the Conservatives are going to make a comparison, they should compare like with like,” he said.

Julia Goldsworthy, education advisor to the Liberal Democrats, said that the Tory figures were particularly misleading because primaries had seen a drop in truancy since 1997.

However, Mr Green said: “I’m happy to trust the Government’s figures. Even if you ignore the 1996 results the trend still shows a big increase in truancy in secondary schools.”

Since 1995 the proportion of half days missed through all absences has dropped from 7.6 per cent to 7 per cent, but the proportion missed through unauthorised absence has remained steady at 0.7 per cent.

The Government failed to hit its target of reducing the half days pupils skip to 0.5 per cent by last year and has set a new target of 0.65 per cent by May 2004.

A Department for Education and Skills spokesman said that truancy was proving a “difficult nut to crack” but that tougher measures had been effective, including jailing the parents of persistent truants.