Heathrow Airports Holdings Ltd has begun the tendering process for work on its £17.6bn third runway, despite the government having not yet given the project the green light.
The airport has called for expressions of interest from firms in four areas of work ahead of main construction getting under way.
These are programme management, information management, construction advice, and design and technical advice.
Interested firms have until Friday (16 October) to respond, after which Heathrow will compile a shortlist and invite these companies to tender in November.
A decision on preferred bidders is expected to be made in the early part of 2016.
The news comes a week after transport secretary Patrick McLoughlin announced that the government would make a decision on Heathrow expansion before the end of the year.
Speaking at the Conservative Party conference in Manchester last week, Mr McLoughlin said: “People have strong views, rightly, but Sir Howard Davies’ commission has produced a powerful report. And we will respond by the end of the year.”
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In July, the Airports Commission recommended Heathrow ahead of Gatwick as the preferred location for airport expansion in the South-east.
The decision was met with strong opposition from some politicians as well as Gatwick Airport, which has questioned Sir Howard’s recommendation on the grounds of air pollution and “complexities in construction” not being fully considered.
It is understood that the decision to begin tendering early has been taken so that Heathrow’s supply chain is ready to begin work as soon as a decision is made by the government.
In July, Heathrow chief executive John Holland-Kaye announced that the airport would begin tendering for work in December.
Speaking at a London First event on airport expansion, Mr Holland-Kaye said: “From the supply chain point of view, we will have a conference in September to outline our plans to the industry and we will be going out to tender probably December or January to get the supply chain ready.”
He added: “We need an early decision from the government and an early vote in parliament to cement that decision.”
Currently, Heathrow is aiming to have spades in the ground by 2020 with a completion set for 2025.