However, the practice has insisted that the plans, drawn up a number of years ago, were only prepared as internal 'option' documents for British Airways (BA) in response to the government's South East Runway Access Study consultation.
YRM admitted it had been asked by BA as early as 1999 to 'consider the masterplanning implications of a third short runway at Heathrow.'
Practice director John Clemow said: 'YRM prepared ideas for British Airways for a short third runway of 2,000m and associated apron and terminal development.
'We believe these ideas helped to inform the Government White Paper 'The Future of Aviation', which was published at the end of 2003 and included the possibility of a third runway at Heathrow if environmental issues were satisfactorily addressed,' he added.
The sensational revelation of the plans in the Mirror arrived as protestors carried out a number of demonstrations around the London airport aimed at stopping its future expansion.