The London Assembly has added more fuel to the debate over airport capacity in the UK by suggesting that runway slots are being left free at certain south-east aviation hubs.

There have been calls for Heathrow - the busiest airport on earth - to be expanded, while some economists want to see a brand new international airport built in order to accommodate the growing number of passengers who want to travel into the UK.

However, the London Assembly believes Stansted, Luton and Gatwick airports are all failing to make the most of their runway capacity.

A report conducted by the organisation showed that 12 per cent of runway slots at Gatwick are currently available and this figure rises to 47 per cent for Stansted and 51 per cent for Luton.

The study also indicated that Heathrow - which is operating at 99 per cent capacity - could still accommodate an extra 20 million passengers a year if larger aircraft are used.

"Evidence we received shows that the Airports Commission must examine whether better use of existing airport capacity could be an intelligent cost-effective alternative to building new airports or runways," commented Caroline Pidgeon, chair of the London Assembly's Transport Committee.