The report, prepared by the sports ministry with the help of outside consultants, dates back to March 2011. Reuters recently reviewed the full report, including its forecasts on air traffic that the government never made public.
"The situation at airports is critical given the current saturation seen in the sector," the report said. "Some airports are in need of urgent solutions."
Senior officials in President Dilma Rousseff's government say the forecasts in the 2011 report are now outdated, thanks in part to major renovations underway at airports in all 12 of the World Cup's host cities.
In an interview, Sports Minister Aldo Rebelo brushed off the grim forecasts, saying airports would have "more than enough" capacity to handle the World Cup, which kicks off on June 12 with Brazil playing Croatia in Sao Paulo.
However, three sources familiar with the 2011 study's methodology and implications say the forecasts are still valid even with the planned improvements.
If they are correct, thousands of visiting soccer fans could face flight delays lasting for hours - a major embarrassment to Rousseff as she prepares to campaign for reelection next October.
If the renovations are not completed on time - a realistic possibility, some experts have said - then the forecasts start looking much worse. Under that scenario, airports would likely face severe operational problems, the report said.
The sources spoke on condition of anonymity because the study was for internal government use only.
Airports have long been considered one of the weakest links in Brazil's preparations to host the global soccer tournament. Fans will need to move around a country that is bigger than the continental United States. Brazil's airports are stretched even on normal days as domestic air traffic soared by double-digits annually over most of the last decade.
Shoddy infrastructure has also become a contentious political issue in Brazil, as improvements to subways and other transport projects have fallen far short of what the government promised when Brazil first won the right to host the Cup in 2007. Economists say the failure to upgrade logistics is a big reason why the economy has struggled over the past three years.
Meanwhile, public anger over poor transportation fueled support for protests that brought more than a million Brazilians into the streets on a single night in June.
The 2011 report, part of a 21-page assessment of Brazil's preparedness for the Cup, contained two sets of forecasts - with planned renovations and without them.
Even with the completed renovations, which range from new terminals to refurbished runways, the forecast said passenger traffic would exceed capacity at six of the 12 airports, and would lead to "critical saturation" at 10 of them.
The airport in the central city of Belo Horizonte was seen as the biggest problem, with forecast traffic at 153 percent of capacity after renovations. Brasilia, the capital, and the northeastern city of Fortaleza were next with traffic expected at 118 percent of capacity.
At those levels, passengers could expect hours-long lines, extended waits for baggage delivery and widespread flight delays, although delays of more than six hours or system-wide flight cancellations are unlikely, the three sources said.
"People will probably still get from Point A to Point B, but it's going to be very uncomfortable, certainly worse than (the government) is saying," one source said.
Without the completed renovations, the report said traffic at the Belo Horizonte airport could reach 260 percent of capacity, while traffic at Sao Paulo's international airport would reach 190 percent.
Under that scenario, "planes will struggle to take off and land," one of the sources said.
Rebelo, the sports minister, said the government will modify schedules to avoid clusters of World Cup-related flights arriving or leaving at the same time, he said. He also pointed out that many foreigners will use charter flights, which are more flexible.
The sources said those factors could help ease the potential crunch but even so, severe overcrowding was still likely.
It is uncertain whether authors of the report considered those factors when developing their forecasts.
Presented with a printed copy of the 2011 report, which was circulated internally just before he became minister, Rebelo shrugged and said: "Those numbers don't match up with mine."
When asked for updated figures, Rebelo said the ministry's press office would provide them.
However, over the next two weeks, officials there were either unresponsive or sent other, non-comparable data. Officials at Brazil's civil aviation department, which oversees airports, also did not respond to repeated requests for comment or updated data.
The 2011 projections were based on air passenger traffic data from the two previous World Cups, in South Africa in 2010 and Germany in 2006, and prepared by an outside, private-sector consultant with experience on similar projects.
Value Partners, a management consulting firm based in Milan, Italy, has examined logistics and other issues related to sporting events for such clients as FIFA, the global soccer body, and major soccer clubs Juventus and Manchester United.
Tina Guiducci, a Value Partners spokeswoman, confirmed the company's role in the study but declined further comment.
Around the time the 2011 report was published, Rousseff took steps to overhaul the aviation sector, including a decision to let private-sector companies operate airports via concessions in several cities - a big switch for her left-leaning government.
Since then, construction has proceeded quickly. In Sao Paulo, Brazil's biggest city and the likely port of entry for most foreign fans, three shifts of workers have toiled around the clock, seven days a week to build a brand-new terminal that the government says will be ready on time for the Cup.
However, it may not be that easy.
A separate sports ministry report, issued in September and made available to the general public, said half of the airport renovations including Sao Paulo were expected to be completed by May 2014 - the month before the Cup matches begin.
That leaves little margin for error, especially given the notorious complexity of building airports. A new airport in Denver opened 16 months late in the 1990s because of a dysfunctional baggage system. Berlin's new Brandenburg Airport, scheduled to open in 2010, is still awaiting its first passenger because of numerous construction problems.
A spokesman for GRU Airport, the private consortium overseeing Sao Paulo's international airport, told Reuters earlier this year that because of the tight schedule the new terminal may see only "partial" operation during the World Cup - in which only a portion of its full capacity is used.