The Bristol-based TV Licensing Authority says 4,000 people were caught watching sets illegally in the city last year - twice as many as in 2003.
The TV Licensing Authority is based in Bristol
The city has been named as the worst place in the south west for licence evaders, with Swindon and Gloucester also in the top four.
Each person found avoiding the £121 fee faces prosecution and a fine of up to £1,000, said a spokesman.
The authority's 20m address database shows all unlicensed properties.
Sara Powell, of the authority, said: "We always prefer people to pay for a TV licence rather than be prosecuted so we are urging all evaders to buy a licence before they are caught."