London Heathrow airport’s operator believes a 3,500m (11,500ft) third runway will provide the greatest benefit to the hub, but it is still analysing whether a slightly shorter one of 3,100m will suffice.

The newly-submitted proposal from the operator, Heathrow Airport, positions the new parallel runway 1,035m north of the current northern runway.

Its length and location would enable all three runways to alternate between arrival, departure and mixed-mode operations while retaining full capacity.

“This flexibility is central to providing predictable periods of respite for communities affected by aircraft noise,” it adds. “This respite could only be delivered by a runway that allows for full operations.”

Its current northern and southern parallel runways, 09L/27R and 09R/27L, have respective lengths of 3,902m and 3,660m.

Heathrow Airport says its third-runway proposal is the result of “extensive design and planning work” on potential alignments, lengths and locations, as well as considerations of cost and negative impact on the surroundings.

But it says it will “continue to discuss and analyse with airlines and other stakeholders” whether a shorter 3,100m runway could provide the same operational and noise benefit.

A rival submission for the hub’s expansion, from Arora Group, has put forward a plan for a 2,800m runway – contained within the east-west length of runway 09L/27R – arguing that it is a simpler exercise, avoiding the need to build over London’s orbital highway.

Heathrow Airport is sceptical of the merits of a competing proposal.

“Introducing alternative promoters would mean missing the government’s timeline for delivery,” it argues. “Not only would other promoters lack the operational site expertise required, but it would mean fundamentally changing Heathrow’s operating structure and the legal and regulatory framework that underpins it.

“It is unlikely that any third party would be able to close its financing arrangements and start main construction works until the regulatory changes are finalised.”

Heathrow Airport also states that rival proposals still require “detailed knowledge” of the hub’s systems, regulatory obligations, and safety needs – elements which are “embedded” in its own plan.

“Any alternative expansion options or standalone elements, such as a runway in isolation, would not possess the operational insight required to identify and accurately cost these factors,” it claims.