Airport security queues of up to four hours cannot be ruled out during the London Olympics.
UK Border Force director general Brian Moore told Parliament's home affairs committee on Tuesday he was satisfied that planned staffing levels were adequate to deal with the expected surge of arrivals, but said he could not guarantee there would be no delays, PA reports.
"I do not anticipate seeing large queues of two, three and four hours because of the work we are doing to move our resources," he told MPs when pressed about recent problems.
"However there will always be circumstances beyond our control."
He repeatedly declined to tell the committee what he personally considered a "reasonable" wait, but said he believed the public generally accepted a 25-minute target.