Montoya was black-flagged after 57 of the 73 laps and retired in the pits while third, and after running as high as second.
The International Automobile Federation (FIA) said in a statement that Montoya, who had a problem on the grid after qualifying fifth, was penalised for incorrect use of the spare car.
"Juan's car was disqualified because...if you decide to change your car on the grid, the driver must be off the grid himself, not necessarily the car, within 15 seconds of the start," said team boss Frank Williams.
"We were off about 13 seconds. Juan is very unhappy about it but I guess you could say we got some mileage on the car for the sponsors."
RULES BROKEN
Montoya and team mate Ralf Schumacher, who crashed heavily on Sunday, were also excluded from last weekend's Canadian Grand Prix after the brake ducts on their cars were ruled to have broken Formula One's technical rules.
Williams technical director Sam Michael said the decision to switch to the spare was taken after the starter would not engage into the back of the car. The disqualification so late after the start was controversial, with many questioning why it took so long for the stewards to react.
"I haven't talked to the FIA people," said Mario Theissen, motorsport director of Williams's engine partners BMW.
"They will probably say that they were very busy with the incidents that happened on the track...but certainly we would have preferred to know immediately what was going on.
"It happened on the installation lap so there was plenty of time even before the start and early in the race to declare that that was not correct.
"The issue itself is clear, it's black and white," he added. "He hadn't left the grid 15 seconds before the start of the installation lap...so that is not open to discussion."
