While some suppliers say they offer “100 per cent renewable electricity”, they do not generate renewable electricity themselves or have contracts to buy it directly from generators, according to Which?

Instead they purchase Renewable Energy Guarantees of Origin (REGO) certificates to match customers’ usage for as little as £1.55.

Which? said the current system allows suppliers who rely exclusively on REGOs to “greenwash” their tariffs while seemingly doing very little to support new renewable electricity generation.

The consumer group found Green Star Energy, Ovo Energy, Pure Planet, Robin Hood Energy, So Energy, Tonik Energy and Yorkshire Energy were all misleading customers with “100 per cent renewable” electricity tariffs.

Green Star Energy, for example, claims to source all electricity from renewable generators, but neither owns renewable generation or has contracts with renewable generators. It purchases REGOs to match customers’ use.

A survey by the consumer group found one in 10 people believe energy firms who sell renewable electricity generate some of that electricity and 8 per cent that they generate all of it.

However, under current Ofgem rules, suppliers selling 100 per cent renewable electricity must have REGOs to prove it. They are not required to generate renewable electricity themselves, nor have contracts in place to buy it directly from generators.

Richard Headland, Which? Editor-in-chief, said: “As consumers grow ever-more environmentally-conscious, it’s concerning that some suppliers appear to be ‘greenwashing’ their energy tariffs, which could risk misleading customers.

“We believe there needs to be greater clarity on how renewable electricity is defined and marketed. People can only make informed decisions about where to buy their energy from if firms are more upfront and transparent about their green credentials.”