The pilot was not arrested and no breath test was done, but the airline suspended him pending an internal investigation. The Federal Aviation Administration (search) is also investigating.
"United's alcohol policy is among the strictest in the industry and we have absolutely no tolerance for abuse or violation of this well-established policy," airline spokeswoman Robin Urbanski said.
Three years ago at the same airport, two American West pilots were arrested after screeners smelled alcohol on their breath before they boarding a plane. Both pilots were fired by America West are now serving prison sentences.
The pilot in Sunday's incident was boarding Flight 1404 to Washington D.C. when Transportation Security Administration (search) screeners thought they smelled alcohol and alerted police, said FAA spokeswoman Kathleen Bergen.
"The officer noticed what appeared to be an odor of an alcoholic beverage coming from the pilot and requested that he exit the aircraft," said Miami-Dade Police Detective Del Castillo said.
The pilot was questioned in the jetway but no breath tests were conducted, Del Castillo said.
Urbanski said the pilot had been suspended pending the outcome of a company investigation.
The flight had been scheduled to leave at 9 a.m. with 76 passengers. According to United's Web site, it took of at 3:40 p.m. after a crew change.