Anti-doping at the Rio Olympics was branded the "worst" ever at a Games on Wednesday night after the process of sample collection was compromised by a wave of no-shows and walkouts by volunteers and testers, as well as a series of security lapses.

Telegraph Sport has learnt that efforts to keep the first Olympics since the Russian doping scandal clean are in danger of being wrecked by the organisational chaos to engulf the Games in Brazil.

The integrity of the entire process was under threat due to major staff shortages, with barely half the volunteers recruited to help run the event reporting for duty and several doping control officers drafted in from overseas walking out after being pushed to breaking point trying to fill the void.

Organisers also admitted that unauthorised individuals had gained access to restricted areas during the drug-testing process, although they denied failing to ensure that anti-doping at the Games was beyond reproach.

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One senior figure in the war on drugs with experience of several major events said of the sample-collection system in Rio: "This is by far the worst I've seen."

Rio 2016 began barely a week after the publication of a damning report in which Russia was found guilty of state-sponsored doping at the 2014 Winter Olympics.

An investigation by Canadian law professor Richard McLaren uncovered a Kremlin-orchestrated sample-swapping plot at Sochi's anti-doping laboratory.

The Rio Games had the opportunity to restore credibility to the drug-testing process at an Olympics but they have done anything but, according to insiders alarmed at how dysfunctional it has become.

One source said that only "very resourceful" doping control officers from countries such as the UK had prevented the integrity of sample collection being compromised.

Another said large numbers of volunteers had picked up their uniforms at the start of the Games never to return, with others reporting for duty each day only to "walk out after being served their free meal".

Some doping control officers were described as having "thrown in the towel" and returned home, amid claims they were being forced to race between venues to collect samples and even pay for taxis out of their own pockets to make it on time.

The director of communications for Rio 2016, Mario Andrada, said he had no knowledge of any problems caused by staffing shortages but did admit volunteers had allowed security lapses to occur during doping control at certain venues.

"In the beginning of the Games, people complained about lack of training of the volunteers," he said. "Not all the corridors leading to the doping areas were cleared. The volunteers were not firm enough and some people were very close to the areas that shouldn't be entered. We corrected this issue."

Rio 2016 admitted that of its 56,000 volunteers - reduced from a planned 70,000 due to budget cuts - only around 70 per cent had reported for duty. Unpaid workers are vital to the successful operation of the sample-collection process at a major event like the Olympics, at which around 5,000 drugs tests will be carried out.

Volunteers are often employed as chaperones to accompany athletes to doping control to ensure tests are not evaded or circumvented in some other fashion.

Michele Verroken, the former head of anti-doping at UK Sport, told Telegraph Sport: "It's hug­ely disappointing that everything that's happened previously around and Olympic Games is continuing to happen. It's just failing our athletes and regrettably making anti-doping looking like it can be incompetent at times. We almost get to the situation where we're lucky to catch anybody."

The Rio Games have already had one scandal related to the drug-testing process when a Kenyan sprinting coach, John Anzrah, was caught carrying the accreditation of Ferguson Rotich when the 800 metres runner was being sought to provide a sample.

Anzrah was sent home but denied posing as Rotich in order to take a test for him, insisting he borrowed the athlete's pass to get a free meal.

It also emerged before the Olympics that Brazil stopped carrying out drugs tests on its leading athletes in the month before the Games after its anti-doping laboratory had its accreditation suspended.

That suspension was lifted in time for the Olympics and, as of yesterday, the lab had processed 3,743 samples from Rio 2016.

Andrada insisted any "glitches" in the process before athletes' urine and blood reached the lab "do not compromise in any shape or form the procedures of the testing".

"We have zero tolerance with doping," he added. "We feel these Games to be clean Games. The doping operation is working as it should be and there is zero risk of an endemic problem or system problem. No way."

Drugs testing at the Games is being monitored by an independent observer group appointed by the World Anti-Doping Agency.

Wada said in a statement: "Wada is not involved in the doping control process [at the Games] except through an Independent Observer mission that will publish its report following the Games.

"The doping control process at the Games is under the responsibility of the International Olympic Committee, which works in collaboration with the organising committee. It is not Wada's role to comment on doping control issues at the Games until the IO Report is published."

The IOC did not respond to requests for comment prior to publication.