BT has signed £146m deal to offer fibre-optic broadband across Scottish Highlands and Islands, which is claimed to be "most complex and challenging broadband project ever".

The Highlands and Islands Enterprise (HIE), which will lead the project, will invest about £12m to complement the £126.4m public funding being offered by the Scottish government, while BT is adding an additional £19.4m into the project.

Upon the completion of the project by late 2016, about 84% of Highlands and Islands homes and businesses will have access to fibre broadband.

Scotland Deputy First Minister Nicola Sturgeon said the new project will connect communities across some of the most challenging landscapes in Europe and provides a platform for future economic development and regeneration.

"Next generation broadband enables businesses to compete on the international stage," Sturgeon said.

"It has the potential to transform the way in which we educate our children, provide health and social care and deliver our public services.

According to the HIE, the geography and dissimilar population of the region present major technological and cost blockades for the commercial launch of broadband and without the initiative, about one in four homes and businesses would have access to fibre broadband.

Under the project, BT will put down 800km of fibre cables on land with 400km of subsea cables over 19 crossings to remote islands, which is claimed to be the biggest sub-sea engineering project undertaken by BT in the UK.

BT Group Next Generation Broadband managing director Bill Murphy said this is a once in a generation project which will deliver massive benefits not only to the local economy but also to local people of all ages.

"High-speed fibre broadband will improve the quality of public services, strengthen the competitiveness of the region’s businesses and protect and develop jobs," Murphy said.

"BT’s network will be open to all communications providers on an equal wholesale basis creating a competitive market where people aren’t stuck with one broadband provider and can choose the package and price that suits them."

The scheme mainly involves fibre-to-the-cabinet (FTTC) services, which are capable of reaching speeds of about 80Mbps, while fibre-to-the-premises broadband will also be launched, which are capable of up to 200Mbps, to some of the homes.