Terror suspects on the Home Office watch list are entering the UK in the runup to the Olympics without the necessary security checks, according to frontline officials at Heathrow.

One senior border officer told the Observer that inexperienced new recruits, deployed to shorten queues after complaints over lengthy waiting times, are repeatedly "missing" passengers of interest who should be referred to counterterrorism officers when they reach passport control.

The official said he was personally aware that three terror suspects – all of whose names are registered on the Home Office suspect index system – had been waved through by staff on his shifts since the start of July. Border officials should immediately notify counterterrorism police or MI5 if they suspect that "SX travellers" are attempting to enter the UK. Another colleague alleged that five suspects were "missed" in one day earlier this month.

"It's all new faces," said the senior official. "The rest of the staff, I have no idea where they have come from, how long they are here for, what their background is. These are people who have been forced by their own department to come here."

The crisis comes days after G4S, the world's biggest security firm, announced it could not provide enough security guards for the London Olympics, forcing the government to call up 3,500 troops to meet the shortfall. Last month John Evans, head of MI5, said the Games offered an "attractive target for our enemies, and they will be at the centre of the world's attention".

The intelligence and security committee also warned that the Olympics had diverted MI5, MI6 and GCHQ from other potential threats to Britain, citing the "vulnerability of the UK at this critical period". It identified potential sources of threat including al-Qaida and its affiliates planning an attack on the Games or participants, especially US or Israeli nationals, and also the possible threat from republican dissident terrorist groups.

Speaking on condition of anonymity, the UK Border Agency official said: "How many other misses have occurred? The missing of counterterrorism 'hits' is a huge thing, but new recruits are not getting enough time to be taught.

"It is vital these people do not get in without being noted and that the information is passed to the police or security services. Once they're in, you've no idea where they might be going."

Many of those drafted in to help cope with border staffing shortages before the Olympics are individuals who have been working elsewhere in the Home Office and have received only basic training to work on airport passport desks. Some have had only a day's training instead of the standard six to eight weeks.

The revelations of lax practice are particularly alarming, coming ahead of pre-Olympic week during which the vast majority of athletes, media and officials will arrive. On 25 July – the day when traffic through Heathrow airport is expected to be most intense – the staffing roster shows, according to union sources, that more than half the employees on duty are from relief staff.

It is understood that counterterrorism police at Heathrow are urgently seeking a meeting with senior UKBA management over the missed alerts. Mark Reckless MP, a Tory member of the home affairs select committee, said: "I know the home secretary would be extremely concerned if the warning index checks were being missed."

Chris Hobbs, a retired police officer who spent more than a third of his service working at Heathrow, Gatwick and in Jamaican airports, said: "Missing passengers who were the subject of security alerts was an extremely rare occurrence. The fact that it appears to be becoming a regular event shows that border controls have all but imploded."

Underlining the inexperience of some of the officials now manning UK borders, a UKBA document recently distributed to staff reassured new recruits that if they did make a mistake their lack of training would be considered.

The document states: "In the event of making a mistake the full circumstances will be carefully considered, including the extent of your training and experience."

Paul O'Connor, Home Office national manager for the Public and Commercial Services Union, said: "Theresa May said she would not compromise border security but you need properly staffed control points with fully trained border officers. They are playing fast and loose with border control."

Last Thursday, the chief inspector of borders and immigration, John Vine, said following an inspection of Heathrow's Terminals 3 and 4, temporary staff were found to process passengers more slowly and ask fewer "probing" questions, satisfying neither the need to deal with lengthy queues nor security concerns.

A border force spokesman said: "All staff being deployed will have the necessary security clearance and will have received the training required to operate effectively. Contingency staff will only be carrying out tasks for which they have been fully trained.

"Contingency staff will operate individually, but are supported by experienced border force officers at all times. If they have any concerns or questions they are told to escalate those concerns immediately."