The Department of Transport confirmed that the Air Accidents Investigation Branch was looking into the incident last month when an incoming British Airways Boeing 747 is believed to have been within a moment of smashing into a British Midlands Airbus A321, which was taking off.
The 747's landing was aborted and the two aircraft were understood to have just missed each other at around 3.06pm on April 28.
A British Midland spokeswoman said: "Our plane was cleared for take off and the BA Boeing was cleared to land. Both aircraft were under the control of Heathrow air traffic controllers."
The issue is likely to raise further concern about the government's planned part-privatisation of air traffic control. According to radio reports, the Heathrow air traffic controller responsible for the planes' movements was reported to be a trainee being overseen by a mentor.
London's congested skies are set to become more so, according to a British Air Transport Association report which today predicted that within five years demand for flights at Heathrow and Gatwick will outstrip capacity by more than 18m passengers a year.
The association's general secretary, Roger Wiltshire, warned that gridlock at the airports meant that the UK was in danger of losing business to other countries.
"While we in the UK are still considering what to do about more capacity, our neighbours in Europe are getting on with it. The extra runways being delivered at Paris and Amsterdam will soon weaken our competitiveness unless we can quickly resolve this critical capacity problem," he said.
But residents and environmental groups are opposed to increasing the number of flights from the airports, as well as the controversial plans to build Heathrow Terminal 5.
The European Court of Human Rights is set to rule today on whether local campaigners are justified in calling on government to cut night flights into Heathrow.
If the campaign is successful, it could lead to challenges against the government from communities under flight paths at other UK airports.
The court case is being brought by eight residents who are all members of the Heathrow Association for Control of Aircraft Noise ClearSkies group. John Stewart, the group's spokesperson, criticised the calls for more flights from London.
"It is particularly unimaginative to call for 'more Heathrow' and 'more Gatwick'," he said. "We need to look at where new airports could be put in the south-east - on the coast, for example, where planes wouldn't fly over people's homes."
An aviation spokesman for the Department of the Environment said the government was preparing a white paper on aviation and airports, but this was being held up while ministers await the findings of the independent inquiry into Terminal 5.
He said: "These things take time unfortunately, but we have to be sure to balance economic, social and environmental needs before deciding what to do about capacity."
