According to UK media, hundreds of passengers were stuck in waiting lines late on Friday and in the early hours of Saturday trying to get into the United Kingdom.
"We are very sorry that passengers faced unacceptable queuing times in immigration due to too few Border Force officers on duty. Border Force were aware of the extra demand from families and we are very disappointed that they did not provide sufficient resource," a Heathrow spokesperson said, as quoted by the Evening Standard newspaper.
The spokesperson added that the airport had brought the issue to the border service and expected it "to provide a better service over the remainder of the weekend."
The UK Home Office also called the situation "unacceptable."
"Border Force is rapidly reviewing its rosters and capacity and flexibly deploying our staff across the airport to improve waiting times," the Home Office said in a statement, as quoted by Evening Standard.
The Home Office promised it was working with the airport and its airlines to resolve the issue.