France's air traffic controllers get more than 30 weeks' holiday a year, winning generous allowances because bosses fear disputes with them, a public audit report said on Tuesday.

Controllers employed by the government's civil aviation division DGAC "work just 100 days a year, as far as we are able to calculate the figure," the audit office said in a report, criticizing the "opacity" of their system of leave.

It said that air traffic controllers work long shifts and are rewarded with long holidays, while managers have a habit of awarding extra days off when operations are overstaffed -- bumping up yearly holiday to more than 30 weeks.

The report said managers keep poor track of this extra time off, which can run to 11 weeks a year on top of the regular 20 weeks of official leave, making it impossible to accurately count the total time worked.

This system of discretionary days off is "unacceptable" and "difficult to reconcile with the security imperatives" of air transport, said the audit office in its annual report, a gauge of spending in France's huge public sector.

The report hinted that the threat of a strike in the security-sensitive sector made bosses reluctant to crack down on the excessive leave system, which is defended by labour unions.