The French invaders

They have flown across the Channel in their millions.

Having already taken hold of Essex, there is nothing to stop them taking over the entire South Coast. Experts say it is impossible to get rid of the hoverfly, a tiny insect which could ruin British beach holidays.

Millions were spotted along the Essex coast this weekend. Worst affected was

the Tendring peninsula, which experienced the highest temperatures in the country on Saturday.

Swarms of the insects descended on resorts such as Walton-on-the Naze.

Andy May of the Essex Wildlife Trust said: "I was sitting on the beach and they are absolutely everywhere. It was amazing.

"There were swarms of thousands of them, flying all around people and landing on them. It seemed like there could be millions of them about. They are attracted to bright colours, so they

are getting confused and swarming round people in bright clothes.

"The flies are absolutely harmless, but really annoying. And there is no way of getting rid of them. We are just going to have to wait for the pollen they feed on to disappear to get rid of them."

He added: "It's strange, because we haven't seen increased numbers of any other insects - it's been a bad year for them, in fact."

The hoverfly is about 1.3cm long and often mistaken for a wasp because of its distinctive orange and black body.

However, the hoverfly plague could be a huge boost for gardens, said Nigel Wyatt, the curator of the Natural History Museum's fly collection. He said: "Their larvae feed on greenfly, so they are actually very beneficial to gardens, eating a lot of pests."

Mr Wyatt believes the weather is the reason for the invasion.

"We've seen massive swarms before. I think the weather conditions are the key. The warm weather and the wind direction probably conspired to bring these over from France.

"Because there are swarms of several thousand of these flies, they are going to be a nuisance. Wearing insect repellent will stop them landing on you, but there's no way of getting rid of them."

He added: "I can see why someone on a beach would find them annoying, but they are actually very good for the environment.

People will just need to put up with them."

Temperatures are set to soar tomorrow, with the hot weather lasting until the end of the week, forecasters said today.

The heatwave that saw temperatures hit 29C at the weekend left Brighton's seafront thronged with more than 100,000 people.

Met Office forecaster Jim Galvin warned: "Temperatures could rise up to 31C tomorrow, but minimum temperatures aren't going to go below 21 for the next few days. That means it is going to be a very sticky, uncomfortable few days in London, particularly at night."

The mini-heatwave is not expected to break until the end of the week, he added.