LONDON -- Blaming a dearth of check-in counter workers and heavy summer traffic, British Airways apologized Tuesday for a backup at Heathrow Airport that prompted cancellation of at least 78 flights over two days and inconvenienced thousands of businesspeople and vacationers.

Enraged travelers denounced the airline, which responded by promising that corporate volunteers and new hires would quickly alleviate the problems.

Tuesday's cancellation of 31 flights was intended to allow the airline to catch up with leftover travelers from the day before, many of whom were stranded by the scrubbing of 47 flights Monday.

A spokesman said the airline, which needs about 2,000 employees at Heathrow at this time of year, was down by about 150 employees this week because of vacancies and illness.

He said the labor shortfall would be eased when 240 people currently being trained start work next month.

The airline had agreed last week to an 8.5 percent pay raise meant to avert threatened strikes of service workers and baggage handlers on Britain's Aug. 30 bank holiday.

Unions say steep cutbacks by management have left employees unhappy and overworked.

Many customers were unsatisfied with the explanations and apologies, and accused the company of penny-pinching and mismanagement.

"BA is becoming a self-serving bureaucracy [with] MBAs left, right and center, but customer service is disappearing down the tubes," groused a customer from Bristol, England, writing to the BBC's web site.