A campaign to map demand levels for superfast broadband in Devon and Somerset will get under way next week ahead of a tender process for a £100 million upgrade.

Organisers of the Connecting Devon and Somerset initiative are urging businesses and residents to contact them with details of their current service so they can demonstrate demand levels to potential private sector investors.

The upgrade has been identified as a key means of unlocking the economic potential of the two counties because it would increase productivity and allow businesses to offer more online services.

"We know it's fundamental to our economic future. We have no hope of attracting new investment, whether it's from companies relocating, people moving here, graduates staying here or new businesses starting up without good connectivity," said Keri Denton, programme director of Connecting Devon and Somerset.

"The majority of rural areas, if they get 2mbps, they are doing well. Most get 0.5mbps which means they can't do simple things like watch BBC I-Player and they probably can't complete their VAT returns online."

Last year, the two counties successfully bid for £31 million from the Government's Broadband Delivery UK agency, a £530 million pot set aside for the so-called "final third" of the country which is unlikely to be upgraded as part of the normal commercial process.

The two local authorities are each investing £10 million with the remaining £50 million to be put up by a private sector delivery partner which is due to be chosen by a tender process which will start in April.

There have been concerns of a growing digital divide in the two counties with urban areas upgraded as part of the normal commercial process but rural areas often having very slow speeds of around 0.5mbps.

The Connecting Devon and Somerset scheme will bring speeds of at least 2mbps to every home and business in the two counties by 2015 and will deliver superfast broadband of more than 20mbps to at least 85 per cent of users by 2015 and 100 per cent by 2020.

With the £132 million Superfast Cornwall scheme, which is backed by European Convergence funding, already well under way, there have also been concerns about businesses relocating from rural parts of Devon to Cornwall to get better connectivity.

The winning private sector firm will be announced in September, with work starting in January 2013 and continuing until 2015.

An exact programme of which areas are upgraded first will be agreed by programme staff and the private sector operator but areas with strong economic potential are likely to be priorities, along with areas with the slowest speeds and communities deemed to have high social need.

More than 26,000 leaflets will be going out to homes and businesses on Monday, urging them to complete details of their current service levels. People can also do this via the website www.connecting devonandsomerset.co.uk or by telephoning 0844 463887.