The UK could lose up to £31 billion in trade by 2030 because of the failure to increase flights to Brazil, Russia, India and China alone, according to research.
The CBI said the study shows the "stark" need to build a new runway at Heathrow, as recommended last week by the Airports Commission.
If there are delays to new runway capacity beyond 2030, the annual cost to the UK economy in lost trade with the so-called "BRIC" countries could be up to an additional £5.3 billion, which will rise each subsequent year, the business group warned.
Welcoming the Prime Minister's pledge to make a decision on the Airports Commission's recommendations before the end of the year, the CBI said it wants to see diggers on the ground by 2020.
Katja Hall, the CBI's deputy director general, will tell a Runways UK conference in London: "Delaying the decision to build a new runway will have a very real economic cost for our country.
"The Commission has been clear in its recommendation to the Government, and so are we - get on with building it without delay.
"A new runway will help rebalance our economy, prevent us handing opportunity to our rivals and avoid a future bill for our inaction.
"When it comes to airport capacity, time is money. We're not just missing out on global opportunities, but paying an economic price right here in the UK.
"Our failure to increase flights to BRIC countries alone will cost the UK as much as £31 billion in lost trade in the period it takes to build a new runway.
"That's just from a lack of flights to the BRIC countries - just the tip of the iceberg.
"We welcome the Prime Minister's guarantee that he will deliver a decision this side of Christmas. He is right too - delaying into 2016 is too late.
"We need to get the legal process under way before the end of the year, and Parliament needs to get behind it. If it does that, we could see spades in the ground by 2020, with a new runway online between 2025 and 2030, and the whole country reaping the benefits by 2040."