Heathrow has warned flights may have to be diverted to other airports if the build-up of queues becomes too great.
The airport has reacted angrily after the Border Force warned last week that passengers arriving in the UK should be prepared for long waits as foreign travel resumes from May 17.
Border Force director-general Paul Lincoln said it could take up to 15 times longer to process people than before the coronavirus pandemic due to the extra checks that will be required.
In a statement, a spokesman for Heathrow insisted that such delays were “unnecessary and completely avoidable” if extra resources were put in place to deal with the additional checks.
However, he confirmed that there were “protocols in place” to divert flights or to hold passengers on planes if the build-up becomes too great.
“Instead of telling passengers to brace themselves for a long wait in immigration queues, Border Force should step up its efforts to automate checks for green list countries, and put in place additional resource for passengers where manual checks might be needed,” the spokesman said.
“Accepting delays is a further demonstration of complacency from them on the matter.
“There are protocols in place to hold passengers on planes or divert them to other airports, to prevent excessive, unnecessary and completely avoidable queues in immigration halls.”
The disclosure is another blow for the beleaguered travel industry disappointed at the limited “green list” released by the Government of countries from which returning travellers do not need to isolate.
The Home Office said that additional staff were being mobilised to minimise queues but insisted that protecting public health had to be the first priority.
A spokeswoman said: “Protecting public health is our priority and as we reopen international travel safely we will maintain 100% health checks at the border to protect the wider public and our vaccine rollout.
“While we do this, wait times are likely to be longer and we will do all we can to smooth the process, including the rollout of our e-Gate upgrade programme during the summer and deploying additional Border Force officers.”