Air travel in India is growing at an incredible pace. The bad news? Its airports can't keep up.

The country's biggest hubs are already operating at 70% of capacity, with New Delhi and Mumbai at more than 80%, according to a new report by the Center for Asia Pacific Aviation. Passenger numbers are likely to exceed capacity by 2022, it said.

Just six airports -- Mumbai, Delhi, Bangalore, Chennai, Kolkata and Hyderabad -- handle two thirds of the country's air traffic, according to government data. Mumbai is already "close to saturation" and is expected to hit its maximum capacity by 2019, CAPA said.

Those cities have earmarked around $5 billion over the coming years for expansion, or to build second airports, but that's only a fraction of how much they will need to avoid bursting at the seams.

CAPA estimates India will need to invest as much as $45 billion on top of that by 2030.

Bangalore, for example, received 2.9 million passengers more in the last fiscal year than it is currently built to handle, and is planning to double its 20 million capacity over the next three years.

Nearly a dozen smaller airports "are already operating beyond" their maximum capacity, the CAPA report added.

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The numbers indicate a looming crisis for India's aviation market, which is projected to overtake the U.K. as the world's third largest by 2026.

Around 265 million passengers passed through Indian airports in the last fiscal year, a number that CAPA projects will rise to nearly 300 million by 2018. India is currently equipped to handle a maximum of 317 million passengers.

Once the planned airport expansions take place, India's maximum capacity will be 463 million. But passenger numbers are projected to continue rising to nearly 700 million by 2027, based on annual growth forecasts of 10%.

Capacity will be exceeded in 2022 at the latest.

"Faster than 10% growth -- 12.5% is more likely -- will result in capacity thresholds being breached even earlier," the report added.

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While the country is set to buy more than 1,000 new aircraft over the next two decades to accommodate the increase in passengers, it may soon run out of space to put them.

The Airports Authority of India, the government body responsible for managing the country's main airports, did not immediately respond to a request for comment.

To make matters worse, only 75 of India's 450-odd airports are currently operational. The government announced plans earlier this year to increase regional air connectivity, saying it would reinvigorate 43 additional airports as long as airlines commit to flying there.