Border Force staff have told ITV News that they are unable to effectively protect the UK's borders amid crippling staff shortages that have left morale at the agency at "rock bottom".
They said they felt ignored and bullied into keeping quiet when they raised concerns about security.
One worker told a survey, conducted by ITV News with PCS Union, that frontline staff were "woefully under-resourced" with more responsibilities being placed on officers day after day.
"Border Force are papering over the cracks," another worker claimed.
"Staff morale is the lowest I have ever known. Staff are concerned about the security of the border, with regard to drug importation, revenue avoidance and people trafficking.
Multiple workers said the focus of the agency was on "keeping queue times down more than protecting our borders".
"Staff are demotivated, angry and resentful of the circumstances they find themselves in," one officer said.
Watch Martin Geissler's report:
The survey of almost 500 Border Force staff at ports and airports found that 98% believed there were not enough staff to check vehicles for illegal substances and stowaways.
Nearly half (48%) said they could not check passports because of staff shortages.
Some 65% said they had been taken away from custom checks to check passports, with 80% claiming this happened daily or weekly.
A Freedom of Information request found that the number of people who managed to evade border security before being identified within the UK had increased by around a third between 2012 and 2013 from 6,520 to 8,564.
Figures also showed that the total expenditure of the Border Agency fell from £596 million in 2012-13 to £467 million in 2013-14.
Mark Serwotka, general secretary of the PCS Union which represents agency staff, said the Government was not providing the resources to protect the UK against illegal substances and people being smuggled into the country.
"Well the risks are fairly clear that stuff is coming into this country that shouldn't, clearly that could they could includes weapons, drugs and ultimately people being smuggled into the country," he said.
"And these are all the things Theresa May says she is tough on, and what this story shows is it's alright to say that but she is not providing the resources to ensure those jobs are adequately done."
Security and Immigration Minister James Brokenshire denied the UK's border security was in crisis, insisting Britain has its most secure borders now than ever before.
He told ITV News' Martin Geissler: "I think we have more secure borders now than we have ever done.
"The work of Border Force is really positive and has made significant strides forward since we came into Government four-and-a-half years ago."
He said the survey did not reflect that the Government had "enhanced" numbers at the agency from 7,300 to 8,100.
Brokenshire said the Home Office had demonstrated how to deliver a more effective service while making "difficult choices" in the current economic climate.
"I want to ensure the public know the focus this Government attaches, at this time more than any, to border security," he added.