Twenty departments (counties) in southwest France are already under a heatwave alert, after temperatures climbed to 37 degrees Celsius at the weekend.

Parts of the country could even reach 40 degrees -- with night-time temperatures everywhere in the low 20s -- before the arrival of storms on Thursday and Friday, forecasters said.

The conditions have awoken comparisons with 2003, when 15,000 mainly elderly people died of heat-stroke and dehydration, and the government was accused of failing to react quickly enough.