Heavy rain and unseasonably low temperatures have marred the beginning of the summer holiday period in many parts of Switzerland, the official Swiss info website reported on Tuesday.

While the mixed weather appears to have had no major impact on tourism, the authorities issued warnings as the levels of several lakes reached a critical point, according to the report.

Several weeks of bad weather with violent thunderstorms and intermittently heavy rain have filled up many riverbeds and lakes, particularly in the region around the capital, Bern, as well as in southwestern Switzerland.

The authorities have called on people living near lakes to watch water levels carefully and evacuate their basements to limit damage to property.

On Monday regions in central and eastern Switzerland were also hit by heavy rain.

A drop in temperatures led to snow falling in the Alps above 1,800 meters at the beginning of the week. In lower-lying areas the mercury climbed to a maximum of 11 degrees Celsius on average.

However, experts have forecast a major improvement for the weekend with temperatures reaching more than 30 degrees. They expect the fair weather to continue for several days.

In April Switzerland recorded a heat wave followed by a period with heavy local thunderstorms. A freak storm in June caused damage worth hundreds of millions of francs.

The hotel industry says the weather has had a limited impact on tourism so far.

Guests who booked their holidays well ahead were less likely to change plans than people who decided at the last minute, a spokeswoman for the Swiss hotel association said.

For their part, farmers are concerned that the wet weather will lead to a poor harvest and difficulties in organizing work.