LONDON (AP) — Prime Minister David Cameron's government delayed a decision on whether to build a new runway at Heathrow Airport on Thursday, arguing that it needs more time to study the impact on air quality, noise and other environmental concerns.

Cameron had promised a firm decision by the end of this year on whether to choose Heathrow or Gatwick for airport expansion. Business leaders in Britain argue that the country needs more airport capacity in southeastern England to keep the country growing.

Heathrow had been selected by an independent panel over rival Gatwick, but in recent weeks, speculation has been building from Cameron's office that a decision would be delayed, in part because of environmental issues raised in a Parliamentary report earlier this month. Cameron's critics also suggested he wanted to delay the fiercely unpopular decision to boost the chances of the Conservative candidate Zac Goldsmith in London's mayoral race in May.

"The case for aviation expansion is clear - but it's vitally important we get the decision right so that it will benefit generations to come," Transport Secretary Patrick McLoughlin said Thursday. "We will undertake more work on environmental impacts, including air quality, noise and carbon."

Further complicating the Heathrow decision for Cameron is a pledge — made when running for election in 2010 — that there would be no third runway at Heathrow "no ifs, no buts." A decision by his Conservative government in favor of the new runway would be seen as an embarrassing reversal.

The move is expected to push back a decision until summer 2016.

The decision comes despite a report from the independent Airports Commission, which concluded in July that expanding Heathrow — already Europe's busiest airport — was the best choice to meet the growing demands of business travelers and tourists. Heathrow and Gatwick, 30 miles (50 kilometers) south of central London, have offered competing projects to build a new runway that will cost as much as 18.6 billion pounds ($29.1 billion).

Business leaders reacted with dismay Thursday to yet another delay on the issue of expansion, which has been discussed, shelved and discussed again for decades. Simon Walker, the director-general of the Institute of Directors, said business leaders will be "tearing their hair out at the news" of a delay.

"At this stage, (our) members care much more about a decision being made than whether the new runway is built at Heathrow or Gatwick," he said.