Scores of air passengers have entered Britain without being checked against a watchlist of terrorists and criminals.
Figures show 134 individuals' names were not run through a key database before they arrived in the UK on private jets or light aircraft in 2014/15.
The number was more than three times higher than the previous year, when 43 were not screened against the Warnings Index (WI).
In an incident in June 2014, some 48 passengers returning to the UK from Europe on one flight were "missed".
The disclosures emerged in an inquiry by the borders watchdog into controls in the general aviation (GA) sector, which covers any aircraft not operating to a specific and published schedule.
It comes weeks after a damning report revealed the WI, which is used to alert border staff to known terrorists and criminals, is hit by two glitches every week.
Pilots and operators of GA aircraft are required to submit a form containing details of their flight and those on board in advance, which is then risk-assessed by officers.
A flight is recorded as "missed" when it has not been physically met or cleared remotely by officers.
Figures showed the number of flights missed despite being flagged as "high risk" fell from 84 in 2012/13 to 20 in 2014/15.
In the last two years teams attended almost 99% of all high-risk flights and cleared the majority of the rest remotely.
Border Force officers were unlikely to attend an arriving GA flight if passengers were classed in advance as "low risk", meaning their passports would not be checked, the report said.
It added: "Some flights carrying passengers assessed as high-risk might also have to be cleared remotely because officers were not available to attend the arrival."
During visits to airports and airfields, inspectors observed that officers attending GA flights were focused on immigration checks and they did not witness any bags being searched for customs purposes.
Arrangements for general maritime (GM), which includes unscheduled sea traffic such as yachts, tugs and small motor boats, were also examined.
Chief Inspector of Borders and Immigration David Bolt described levels of knowledge and understanding of threats and risks as "generally poor".
He said: "Both GA and GM pose significant challenges for Border Force and for the other agencies involved in managing threats to the security of the UK.
"This report identifies a number of areas where Border Force needs to make improvements, particularly in relation to GM."
Immigration minister James Brokenshire said border security "remains our priority".
He added: "We have a clear vision for the future of general aviation and general maritime security which builds on significant improvements delivered in the last year.
"But we are not complacent about the security challenges we face in these sectors - as an island nation with 11,000 miles of coastline we have to maintain vigilance at all times.
"We use an intelligence-led approach and work closely with other law enforcement agencies."