Britain's top weather experts are to crack down on rogue forecasters who sell substandard predictions of sunny periods and heavy showers.

The Royal Meteorological Society (RMS) says it wants to protect the public from unqualified firms that issue sloppy forecasts to people planning events such as weddings and holidays. Experts say the number of such companies has grown rapidly recently, and the society is worried their poor predictions could undermine already shaky public confidence in weather forecasts.

Dr Liz Bentley, head of communications at the RMS, said: "There are cowboy operators out there. Lots of people have set up on the web and push forecasts out, but not only do they have poor-quality data, their people are not qualified." The society is working with several large forecast firms to draw up voluntary standards for the industry.

"We want to set a minimum bar that people have to meet, to make sure the public can have confidence in weather forecasters," Bentley said. "The level won't be that hard to meet. We don't want to squeeze out newcomers and minimise commercial activity, but we have a duty to protect people. We have to get the balance right."

The society is still examining how its plan would work, but one option is a kitemark-style badge that approved forecasters could display.

Chris Blowes, director of forecast firm Weather Commerce, which is involved with the planned RMS standards, said: "There are now lots of amateurs in this game who are very good at IT and can come up with all sorts of kinky websites that produce automatic forecasts, but they're being a bit naughty, really, as you couldn't use those forecasts to plan anything."

Many sites source their weather data from the US National Weather Service, which must place them into the public domain under federal laws.

Blowes says: "There is a difference between weather data and a weather forecast. You can have as much numerical data as you want, but if you can't turn it into a reliable forecast then you shouldn't be in this game."

Pete Inness, a lecturer at the Department of Meteorology at Reading University, said the results from a single weather centre, such as the US service, are not always reliable.

"Each has its own [weather] model that can produce a slightly different forecast and at other times can produce very different forecasts," he said. "If five or six models all say the same thing, then you can have quite a lot of confidence in your forecast. But sometimes three can say one thing and another three say something totally different." Forecasters must then step in to make sense of the conflicting information.

Bentley says the planned RMS standards will ensure that forecasters must demonstrate these skills, perhaps through a NVQ qualification. Those who work on high-altitude weather for airlines must already hold professional chartered status.

The RMS move follows reports that the BBC could drop an underperforming Met Office, which wrongly predicted a "barbecue summer", from its bulletins and has invited other forecasters to bid for the contract for the first time.

One source at a rival company said: "We don't think the BBC will ditch the Met Office, but this is a warning for them to get their act together."

Inness says that short-term forecasts are generally better than the public believe. "The problem is that people don't really listen to the weather forecast properly. It comes on at the end of the news and it's on in the background like wallpaper. They might hear the odd word like snow, but unless they are really paying attention they can very easily get the wrong end of the stick."