A whistleblower who works at Heathrow Airport has told Sky News the system is at breaking point.
The immigration officer said it is "frightening" how many people slip through the net and exposed a culture of deceit where employees are bussed in when politicians visit to give the impression of more staff.
He told Sky News he feels security has been "compromised" because of pressure over queues.
The whistleblower, who wishes to remain anonymous, said: "We've operated a light touch over the summer for the past couple of years.
"It was never meant as an intelligence-led way of improving security. It's all about the queues."
Home Secretary Theresa May is fighting for her political reputation after it emerged checks on people entering the UK were secretly relaxed over the summer.
She has blamed the former head of the UK border force Brodie Clark for acting without ministerial approval.
Mrs May approved relaxing certain checks on passengers within the EU but accused Mr Clark of going much further.
The Government's pilot aims to deploy resources more effectively by using intelligence-led risk assessments.
But the Terminal 3 worker strongly disputes any suggestion that the looser checks he has witnessed are a better use of resources.
He said: "It's just a way of trying to get people through passport control quickly. It only saves 30 seconds per person, but that adds up to a lot.
"We would only use light touch checks when there were big queues - from around 2.30pm onwards over the summer months.
"For instance, if there was a big flight arriving from Pakistan.
"It makes us very angry because that's the very time when we should be taking time and asking questions.
"But the truth is, there's not enough staff. We can't cope with the number of people.
"There's only five or six of us and we have the worst of the queues at Terminal 3."
The whistleblower does not believe Mrs May was aware of what happened or that she ordered the relaxing of the controls.
He said: "I think Theresa May has been misled by Brodie Clark. He never implemented what she thought. But he may not have been aware of what was happening on the frontline either."
But he also has concerns over aspects of the pilot ordered by Mrs May, in particular the flexibility over relaxing checks on children.
He said: "Operating a light touch with children is serious. It's very dangerous.
"If there's a family with four children in tow coming off a long-haul flight, one of those children could have easily been slipped in with that family."
Sky News has asked Heathrow airport if it would like to comment on the whistleblower's claims but it has yet to reply.