Security guards working for Serco, the private contractor drafted in to protect Britain's borders during the Olympics, have been at the centre of a series of security breaches that have allowed vehicles to enter the UK without screening for "dirty bombs".

Concerns about queues and job cuts at the UK Border Agency forced the Home Office to bring in private guards to man radars designed to detect radiation at major ports. But Serco staff who took on the roles once occupied by public sector officials in mid-May have been at the centre of security scares.

It is understood that the border force is looking into the failure of a Serco guard to alert engineers when a security screening system went down at Dover ferry port, allowing vehicles to enter unchecked for radiation.

Serco staff have been repeatedly reported to managers for failing to correctly use equipment that checks for signs of radiation underneath cars and lorries. Border force officials admit that this is because staff had "been provided with training some time ago, they have become unfamiliar or not previously had hands-on experience".

Some of the 300 Serco staff "left areas unmanned" despite the heightened security threat facing the country weeks before the 2012 games. Serco staff have also missed alarms that are expected to prompt secondary searches of vehicles, the Observer understands, although it is understood that such "misses' also happened before the private contractor, whose turnover last year exceeded £4 billion, became involved.

More than 320,000 foreign visitors are expected to visit Britain during the games, which begin in 19 days. As a result of cuts announced in October 2010, the Home Office is on course to shed a third of its workforce by 2015. UKBA, the department's largest agency, is well on the way to cutting 22% of its staff – 5,300 jobs.

The influx of people has caused mayhem at some borders, with queues at Heathrow airport lasting some two and a half hours in recent weeks. In response, 600 civil servants and former immigration workers are being drafted in to help, at a cost of more than £2.5m.

The radiation detecting equipment was set up as part of Programme Cyclamen, an initiative between the Border Agency and the Home Office that forms a key element of the UK's counter-terrorism strategy. Cyclamen screens sea and rail traffic, including freight, vehicles and passengers. If an alarm is activated, Border Force officers are expected to use handheld devices to investigate. the source of the radioactivity.

A Border Force spokesman said: "The security of the UK border remains our top priority and all staff are security cleared and fully trained to operate their specific role. Serco staff work alongside Border Force officers and will be expected to adhere to the same high standards as our own staff."