By Sarah Young

LONDON (Reuters) - Britain should only approve Heathrow Airport's plan to build a new runway if it keeps fees flat to protect consumers, a parliament committee said, as it made recommendations on air quality and noise ahead of a parliamentary vote.

British lawmakers will vote on whether to give the expansion of Heathrow the green light before July, but first the policy details on which they will vote must be finalised.

Publishing its recommendations, the Transport Select Committee said the government needed to add safeguards to its Airports National Policy Statement to make sure passengers, communities and the environment were protected, and help ensure it finally gets built after decades of delays.

"The committee's recommendations improve the NPS and reduce the chance of a successful legal challenge," the chair of the committee Lilian Greenwood said on Friday.

Such challenges are likely given the sensitive nature of the expansion plan for Heathrow, whose flight paths affect densely-populated London, causing opposition due to complaints about noise and air quality.

Heathrow has estimated the bill for expansion at 14 billion pounds, which lawmakers say will make it one of the largest privately funded infrastructure projects in the world.

Heathrow's biggest airline British Airways, plus other airline customers, have been vocal in demanding that the airport does not hike its fees to make them pay for it, as this will risk tickets becoming more expensive and the new facilities not being fully used.

The committee agreed and said this should be formalised in the NPS.

"We recommend a condition be included in the NPS that airport charges be held flat in real terms but with scope for a marginal increase provided the balance of benefits is in favour of the consumer," the report said.

British Airways owner IAG said it welcomed the report.

Heathrow has previously committed to keeping charges close to current levels. "We are ... clear that we will deliver an expanded Heathrow that is sustainable, affordable and financeable," the airport said in response to the report.

The committee also asked for more evidence on the affordability and financing of the expansion, and the steps being taken to prevent the bill for the project rising.

Transport Secretary Chris Grayling has said the government aims to give the formal go-ahead in the first half of 2018, subject to securing parliament's backing, with the work expected to start in 2021 once planning permission is secured.

Heathrow is owned by Ferrovial, Qatar Investment Authority and China Investment Corporation among others.

(Reporting by Sarah Young, editing by David Evans)