Ofcom has announced a new investigation into the mis-selling of fixed-line telephony services.

The regulator last investigated the issue three years ago, and found that many companies engaged in a variety of dodgy practices.

These included 'slamming', where customers are switched to a new provider without their knowledge or consent, along with making it difficult to switch providers and even pretending to be a different company.

Ed Richards, chief executive of Ofcom, said: "Far too many consumers are being misled by fixed-line companies and this investigation will take firm action against those who disregard the rules.

"We want to make sure that consumers are treated fairly and are able to take full advantage of the increasing choice and lower prices that competition is delivering."

Ofcom took action against eight companies during its last investigation, and levied its maximum penalty of a fine equating to 10 per cent of turnover against one provider.

As a result, complaints about mis-selling fell from 1,200 a month in 2005 to around 350 a month in January 2008.

The new investigation will focus on telecoms companies that have refused to join independent arbitration services such as Otelo or Cisas, despite Ofcom's ruling that they must.

The regulator has promised to take action against those companies that refuse to join.