Broadband speeds of up to 100 megabits a second were yesterday promised as part of a "bold" £1.5bn investment by telecoms giant BT.

But, amid uncertainty over which parts of Scotland will be able to take advantage of the development, BT urged the Scottish Government and Highlands and Islands Enterprise to help ensure rural areas can gain access to it. The programme could see as many as 10 million UK homes connected to a new fibre optic network by 2012, instead of the copper wires that link to most consumers.

BT said the roll-out will allow households to run a number of bandwidth-hungry applications at the same time, such as high-definition movies, gaming, complex graphics or video projects.

The "10 million by 2012" target is dependent on BT and other service providers being allowed by Ofcom, the telecoms industry regulator, to charge enough to justify the huge investment.

If it gets the green light, BT said it would focus its investment in places where there was a strong public-sector interest in partnering with the company and using its new services.

The programme would be the largest of its kind ever seen in the UK. Much of the country is already set to receive advanced new copper-based broadband services of up to 24Mb/s - up to three times the headline speeds currently available for most homes and businesses. The last 21 exchanges in the Western Isles still to be upgraded for broadband will be included in this wave.

Now BT is looking even further ahead to the age of superfast fibre optic-based broadband but it is not known how much of Scotland will have access.

Brendan Dick, director of BT Scotland, said: "I have already talked to Highlands and Islands Enterprise and what I want them and the Scottish Government to think about is how we can work together on this.

"To some extent in the big urban areas the market will look after itself, so we will need to stimulate demand and aggregate it across our geography.

"If we do that we can make a case for this superfast broadband whereas if we don't there are bound to be some parts where it would be much harder if not impossible to make a case.

"Scotland is effectively competing for a pot of money that is going across the whole of Britain."

He added: "BT is keen to invest further in Scotland so I am urging the Scottish public sector to work with us to ensure the funds are directed to our country."

Alistair Carmichael, LibDem MP for Orkney and Shetland, commented: "It is ironic that the people who would stand to benefit most from superfast broadband are the people in our more geographically remote communities but we will probably be the last to get it.

"There is a real job for the Scottish Government to get behind this. If the warm words of sustainability mean anything then this is exactly the sort of thing they must help fund. They must understand the market is not going to provide fibre optic cable to every house in the west of Shetland."

Shares in BT fell 5% yesterday as it suspended a buyback of shares because of the need to invest £1.5bn in the project.

Rivals offering broadband through the "local loop" from the telephone exchange to the home were also impacted by BT's announcement, with Carphone Warehouse shares down by as much as 8%.