Herbert Smith has won a high-profile mandate to advise airport operator BAA on the controversial third runway at Heathrow and a £4.3bn development project at the airport.

The top 10 City law firm saw off competition from more than 50 firms to secure the coveted instruction at the end of last year, after BAA publicly advertised for a legal adviser for the projects.

The appointment will be seen as a coup for Herbert Smith's 13-lawyer planning team, given the size of the development programme.

BAA is expected to put £4.3bn towards redeveloping the airport, including building a new Terminal Two. This figure does not include the cost of the third runway project, which is expected to involve an investment from BAA of up to £9bn.

Herbert Smith has worked with BAA for more than a decade, with City planning partners Matthew White and Patrick Robinson leading the relationship.

BAA has previously worked with a number of other City firms including Freshfields Bruckhaus Deringer, which most recently advised on BAA's £935m bond issuance and the £1.5bn sale of Gatwick Airport to Global Infrastructure Partners, both of which closed in December last year. Other firms to be instructed by BAA in the past include Lovells and CMS Cameron McKenna.

BAA would not comment on the value of the legal contract but confirmed Herbert Smith's appointment.

SJ Berwin planning partner Simon Ricketts said: "It is potentially a huge project and a very complex one which will need years of work, and it is also not uncontroversial. It is one of the largest planning projects out there at the moment."

Herbert Smith on the Legal Week Wiki