Heathrow Airport Holdings is set to kick off a tender process to appoint external legal advisers, after it was cleared to push ahead with plans to bring the third runway at Heathrow Airport into operation.

Berwin Leighton Paisner (BLP) and Pinsent Masons have been working alongside the airport's in-house legal team on work to date, but the organisation is now set to launch a formal tender process to appoint legal advisers for the next stages of the process.

It is understood that Heathrow is also intending to hire a head of legal for planning to help oversee the project.

Both Pinsents and BLP are on Heathrow Holdings' sub-panel for planning and property, alongside Eversheds and Herbert Smith Freehills.

The BLP team is being led by planning partners Tim Smith and James Good, while Pinsents team is being led by infrastructure planning head Robbie Owen and planning partner Francis Tyrrell.

DLA Piper has also played a role in the process, with a team led by the firm's head of infrastructure Colin Wilson advising the UK government's Department for Transport.

The plans come after the government yesterday (25 October) gave the controversial third runway the go-ahead, despite opposition from rival airports such as Gatwick, environmental groups and Members of Parliament from constituencies that will be affected by the expansion.

One City planning partner said the primary role for external legal advisers at this stage would be the promotion of the planned scheme, which would account for "80%-90% of the work".

The partner said there would also be a requirement for advice around "policy and policy promotion".

Objectors to the new runway have already started preparing legal challenges.

Environmental campaign group Greenpeace has teamed up with the local councils of Hillingdon, Maidenhead, Richmond, Wandsworth and Windsor to contest the government's decision.

The group has hired London law firm Harrison Grant, which specialises in public, environmental and planning law, to prepare a judicial review.

In 2010, Harrison Grant environmental and planning partner Kate Harrison acted for a coalition of local authorities, environmental groups and residents that mounted a legal challenge and successfully blocked a previous attempt to build a third runway at the airport.

A judge in London's High Court agreed with the plaintiffs' assertions that the project's environmental impact had not been sufficiently investigated and ordered the government to undertake a formal review of the policy.

DAC Beachcroft has been advising rival scheme Heathrow Hub, a consortium of private investors that proposed an extension of the north runway at Heathrow.

The firm's head of planning, Christopher Stanwell, said: "We continue to work with Heathrow Hub Limited and its consultant team," and added: "We will be assessing the reasoning behind yesterday's decision and proposed consultation process on airspace and the draft National Policy Statement."

Nabarro infrastructure group head Kristy Duane is also acting for Heathrow Hub on corporate matters.