A special comment report by Moody's Investors Service reveals that adding a new runway at either Heathrow or Gatwick would have conflicting credit implications for London's three largest airports, namely Heathrow, Gatwick and Stansted.
The report said that Gatwick will face long-term financing risks, irrespective of whether a new runway is built there or at Heathrow.
Moody's has assessed the implications across all three airports ahead of the final recommendation by the Airports Commission, which is supposed to be released by mid-2015.
Moody's vice-president, senior analyst and author of the report Xavier Lopez del Rincon said: "A new runway will have mixed credit implications for London airports.
"A runway at Heathrow would allow the airport to benefit from growth in future traffic volumes and a new runway at Gatwick would not take significant traffic from Heathrow. Gatwick, on the other hand, would be vulnerable to airlines switching to an expanded Heathrow, whilst a new runway at Gatwick would increase its airport charges and could alienate its price-sensitive airlines."
The report also stated that a runway at Heathrow would enable the airport to accommodate the estimated growth in London passenger traffic. Heathrow will be able to handle between 133 and 149 million passengers by 2050, which is almost double the current capacity.
Although Gatwick's proposed second runway will cost around £7.4bn, which is around half as much as the two Heathrow schemes under consideration, the report notes: "The financial risks associated with the scheme are high, given the size of the project in relation to the company's size and Gatwick's relative lack of experience undertaking such a transformational capital expenditure programme."
The report claims that Gatwick will be more vulnerable to competition if a new runway comes up at Heathrow as it would risk losing scheduled airline traffic, where carriers can typically earn more per passenger mile.
However, the construction of a runway at Gatwick will lead to almost doubling of aeronautical charges at the airport, which would put Stansted at a huge disadvantage.
According to the report, a Heathrow runway would not affect Stansted, as the latter specialises in servicing low-cost carriers, which are entirely absent from Heathrow.