The British airport operator vying with Citigroup- and GE-linked funds to buy London's Gatwick Airport has drafted in one of Britain's biggest retirement funds to strengthen its bid.

Manchester Airports Group (MAG), already working with Canadian infrastructure investor Borealis, has brought in Greater Manchester Pension Fund (GMPF) as a minority partner in the consortium, a source familiar with the matter said on Friday.

The fund manages public sector pensions for the 10 local authorities of Greater Manchester in northwest England, who also own MAG. At GBP9.23 billion pounds (USD$13.5 billion) in 2008, it is one of Britain's largest funded pension schemes.

The MAG-Borealis group is one of three consortia preparing binding bids for Gatwick, Britain's second-busiest airport, which could be worth up to GBP2 billion. Bids are due by the end of April.

The group's bid is likely to be cautiously structured, with up to half the value funded by equity, the source added.

Gatwick is part of the seven-airport British Airports Authority (BAA) group majority owned by Spain's Ferrovial.

Gatwick was put up for sale last year to meet competition concerns and BAA has since been formally ordered to sell it and two other airports.

But two bidding groups have withdrawn, blaming a high asking price and scarce debt, and an earlier sale deadline was delayed to give bidders more time to agree debt finance with banks and conduct due diligence.

The MAG group is competing with Global Infrastructure Partners (GIP) -- a fund with General Electric and Credit Suisse as founding investors that already owns London City Airport.

The third consortium is Citigroup unit Citi Infrastructure Investors, with Vancouver Airport Services and John Hancock Life Insurance, who are bidding jointly as Lysander Gatwick Investment Group.