WASHINGTON – Airfares are likely to stay high throughout this decade, as passenger travel grows but airline capacity shrinks, according to a government forecast issued Thursday.

In its annual economic analysis, the Federal Aviation Administration predicted that more airline mergers and consolidation will shrink the number of cities served.

The combination of modestly increasing demand and shrinking capacity means that planes, already more than 80 percent full, will get even slightly more full and remain that way for the next 20 years, the agency said.