Scotland is losing out on £1 billion of investment that could revitalise its struggling transport infrastructure, the SNP has claimed.

The party's transport spokesman, Kenny MacAskill, claimed that the Treasury is failing to invest revenue from air passenger duty collected at Scotland's airports.

He claimed duty levied on flights from Scotland would bring in at least £1 billion between 1999 and 2010 with no substantial increase in passenger numbers.

But if numbers do increase as predicted there would be enough cash to fund the direct rail links proposed for Edinburgh and Glasgow airports.

"We have no alternative in many circumstances due to our geography but to fly and often to take two flights as we lack a direct connection," said MacAskill.

"We accordingly more than pay our share in taxes. However we don't see the benefit in infrastructure or improvements.

"These sums are enormous and if retained and utilised here could really see aviation within and without Scotland take off."