Passengers arriving at Heathrow faced another day of long queues to pass through immigration checks.
One traveller wrote on Twitter that a queue for families with children was three hours on Tuesday.
Another described the queue for families as “shameful”, adding that the UK “must be the only country which treats families worse than adults”.
Some said there were not enough Border Force officials.
On Monday a passenger said the immigration process in Terminal 2 was “incompetent, ridiculous”, adding that he was forced to wait for more than five hours with “no water, no bathroom”.
Heathrow wrote on Twitter that Border Force is “currently experiencing some delays as they conduct additional spot checks to ensure passenger compliance with the UK Government’s latest entry requirements”.
It went on: “Waiting times at the border have on occasion been unacceptable and we have called on the UK Government to address the problem as a matter of urgency.”
A Home Office spokeswoman said: “Our utmost priority is protecting the safety and health of the public and we will never compromise on security, and on ensuring passengers are compliant with the current health measures, which means passengers will need to accept an increase in the time taken to cross the border.
“The rollout of upgrades to our eGates to automate checks for health requirements is ongoing, with many eGates already in operation and more to be added over the coming months to increase automated checks on passengers at airports.
“However, for safeguarding reasons families with children under the age of 12 are not permitted to use the eGates.
“Where there are high volumes of families with young children, such as over the summer holidays, Border Force may dynamically deploy resources to frontline desks instead and we continue to flexibly deploy our staff to make the process as smooth as possible.”