The Home Office has called a report "inaccurate and highly misleading" after it estimated more than 4,000 'high-risk' flights per year do not face proper security checks.
The research from the right-leaning Adam Smith Institute also warned that Britain's Border Force is stretched to "breaking point".
The authors claimed spending per passenger is down 25% and morale in the force is at "an all-time low", with staff reporting that they do not have the resources needed to do their job properly.
Sam Bowman, the executive director of the Adam Smith Institute, added: "A successful Border Force needs to do two things: keep people out of the country who should not be allowed in, and do so without causing unnecessary disruption to other passengers.
"In both these respects the Border Force is not succeeding.
"Its security systems are out of date, overstretched and failing to cover all passengers adequately. It's astonishing that potentially thousands of high-risk flights are not being checked properly by the Border Force."
But a spokesperson for the Home Office said: "Multiplying the number of flights missed over two random days to reach a speculative figure for the whole year both is inaccurate and highly misleading
"Border Force carries out detailed risk assessments and our officers physically meet any flight considered to be high risk. In addition, we work with domestic and international partners to gather intelligence and target suspicious activity in the skies and seas.
"We constantly review our approach so that we are keeping up with the ever-changing methods used by criminals looking to bring harm to communities in the UK."
Key claims of the Adam Smith Institute report
- British Border Force in complete disarray after years of neglect
- Rising passenger numbers straining stretched force to breaking point
- As many as 4,197 high-risk flights not physically met by Border Force each year
- Brexit call for tighter controls over immigration must be met
- Failed reform attempts have cost the taxpayer £1bn
- UK needs fully revamped computer system to secure borders