Customs checks aimed at thwarting drugs and arms smugglers have been downgraded in order to deal with the growing queues at Britain's major airports, according to frontline officials.

Senior immigration officers and border force unions say staff shortages and growing political pressure to reduce queueing times mean that operations to combat the influx of drugs, guns and other contraband into the UK have, in effect, been placed on hold.

One senior official at Heathrow told the Observer the situation was so serious that Britain's busiest airport could be described as having "no border controls" when it came to smuggling. The officer, who wished to remain anonymous, added that passengers identified as suspicious, including those accredited to work on Olympic sites, were being waved through without extra security checks because they there were not enough staff to tackle queues.

Last week it emerged that the UK Border Force was failing to meet its Heathrow passport control targets on an almost daily basis. Unions confirmed the "effective abandonment" of customs operations at airports because staff shortages meant all available personnel were being pulled on to passport control. Paul O'Connor, Home Office national manager for the Public and Commercial Services Union (PCS), which represents 6,000 border staff, claimed that customs officers believed the huge cutback in operations was already leading to an increase in small-scale drug-smuggling into the UK.

The senior official at Heathrow, who has almost 10 years' experience, said: "We have actually ceased doing [anti-smuggling operations] at the moment, even though they won't say they have. Word has already got around to criminal enterprises."

Chris Hobbs, a former Metropolitan police officer who worked with border control at Heathrow and Gatwick before retiring last summer, said: "Organised crime networks will only be too well aware of this and, if they can recruit couriers, will be having a field day."

A Home Office spokesman said: "We are committed to maintaining border security. By deploying our staff flexibly we are continuing to target drugs and illegal weapons while carrying out our immigration work as rigorously and efficiently as possible."

The shadow home secretary, Yvette Cooper, urged Theresa May, the home secretary, to "stop hiding" and investigate the escalating airport security "shambles". Cooper said: "She needs to make sure that appropriate customs checks are still taking place and that panic action to staff the passport desks is not leaving customs completely empty."

The PCS also warned that extra baggage checks and detailed questioning of suspect passengers were not being carried out because staff had been moved to passport control. Security concerns among border staff, O'Connor said, had been raised repeatedly with line managers and with May, ahead of the Olympics, but to no avail.

The frontline Heathrow border official described an incident last week involving two students from Pakistan, both with Olympic accreditation. He felt they posed a potential security risk, yet they were allowed to enter the UK without being challenged or having their bags searched.

He said: "One was already green accredited which means he was going to be working on the Olympic site somewhere. I don't know what venue, I just know he's been passed as secure. It's shocking, they're landed and there is nothing I can do. It's an accident waiting to happen."

The pressure to cut queuing times was so acute that he had stopped "running cases" – questioning suspect passengers on their travel history, their intentions in the UK and examining their baggage – because passport control was too short-staffed.

Last year, he said, his team was running up to 50 cases a day. "And that was very light compared to a few years ago. I would normally have done three cases a day myself on a shift. I can't even do one at the moment if I want to because I don't have the time."

Hobbs, author of the novel Olympic Flames 2012, in which a riot takes place at the London Games, said the situation raised questions over the vetting process for the Games and the broader approach to border security.

"Border officers are encountering foreign visa-holding students re-entering the UK whom they are less than happy with. On checking UKBA databases, they are finding these students have been given accreditation to work at Olympic venues, including the Olympic Park.

"Requests to carry out further inquiries in respect of these passengers are normally refused by chief immigration officers due to the fact there are huge queues and insufficient staff."

Cooper said that May needed to explain to parliament why such a security risk had been allowed.

"The home secretary was warned about the risk of cutting so many border staff, particularly in Olympic year," she added.

Figures detailing airport queuing times – on Saturday, delays at Stansted passport control were branded "unacceptable" – have intensified the pressure on May, with the prime minister already asking her to explain the problem. The situation is likely to be compounded by a threatened one-day strike by border staff. the home secretary's insistence on full passport checks is fundamental to the fiasco.

The former head of the UK border force, Brodie Clark, lost his job last November after relaxing certain passenger checks to cope with queues while preserving security checks on suspect travellers.