It will be difficult to get most Brazilian airports ready in time for the 2014 World Cup, according to a government study released Thursday.

Renovation work in 10 of the 13 airports which will be used during the monthlong tournament will not be completed in time unless there are changes in the investment plans made by Brazil's airport authority, Brazil's Economic Research Institute said in its study.

"If the usual timelines for infrastructure projects in Brazil are maintained, 10 of the 13 passenger terminals will not be completed by 2014," the report said, adding that "the low efficiency in the execution of investment programs is worrisome."

The study shows that even if the upgrades are finished, most airports would remain operating over their capacity because Brazil's strong economic growth is expected to significantly increase passenger demands at the airports in the next few years.

"Ten (airports) would be operating in critical situation," the study said, adding that currently 14 of Brazil's 20 biggest airports are already operating over capacity.

Brazil's woeful airports infrastructure has always been one the main concerns for local officials trying to prepare Brazil to host soccer's showcase competition three years from now. Stadium construction and improving public transportation on the ground also are challenges facing Brazil.

The Brazilian soccer federation and some government officials had already said they were worried with the slow pace of upgrades at the airports. A government watchdog group also warned last year that renovations in some airports may not be completed by 2014.

Brazilian President Dilma Rousseff said recently that upgrades to the country's airports will be opened up to private investment to ensure they are finished. She said the government would make "a strong intervention" to fix the nation's overcrowded airports, including creating a civil aviation ministry to oversee the country's civil aviation agency and the nation's airport authority.

Brazil's infrastructure investments for the World Cup will reach nearly $20 billion, including about $3 billion on airports, which need significant upgrades to accommodate the 600,000 visitors expected for the World Cup.

Brazil will also host the 2013 Confederations Cup and the 2016 Olympics in Rio de Janeiro.