Sunny weather is set to return to Britain in time for the start of the London Olympics after weeks of unseasonable summer downpours, forecasters predicted on Wednesday.

The Met Office, Britain's national weather service, said some much anticipated sunshine would return on Sunday and that southern England would enjoy dry weather next week, in time for London's opening ceremony on July 27.

"Confidence is steadily increasing for the return to normal conditions to continue through the final week of July," the forecaster said.

"This weather pattern, for the UK as a whole, is much closer to the climatological norm than the cool and wet scenario experienced so far this month."

Forecasters have blamed the downpours on the jet stream, a strong-flowing ribbon of wind that crosses the Atlantic, settling unusually far south, but they predict that it will move northwards again soon.

But the Met Office warned that the weather will remain "changeable" in the weeks ahead.

"The south is most likely to see the best of any dry, bright, and at times warm weather, particularly at first," it said.

"Some rain is likely at times but overall, conditions are unlikely to be as bad as we've seen so far this summer.

"However, a lengthy spell of hot, sunny weather does look unlikely."

Britain has suffered widespread flooding in recent weeks, and the wettest period of April to June on record.

Arriving in London on Monday, the first overseas Olympic athletes were greeted by leaden skies and persistent drizzle.

Chinese athletics star Liu Xiang was among a number of athletes who left London to complete his pre-Olympics training in Germany because of the wet weather, a report said Tuesday.