Senior figures within amateur boxing have warned many bouts, including those to decide medals, could be fixed at the Olympic Games in Rio de Janeiro amid widespread concern about corruption and financial malpractice at the sport’s global governing body, the Guardian can reveal.

Horrified senior officials within the sport believe a cabal of officials are able to use their power to manipulate the draw and the judging system to ensure certain boxers will win.

One senior figure said there was “no doubt” some of the judges and referees in Rio “will be corrupted”. He alleged a group of referees get together before major championships to decide how to score certain bouts.

Almost every recent Olympics has featured controversy. Most infamously, at the 1988 Games in Seoul the home favourite Park Si-hun beat the American Roy Jones Jr in a light middleweight contest the judging of which became a byword for scandal.

The Romanian former International Boxing Association (AIBA) vice-president Rudel Obreja alleged manipulation of the judges’ draw at the 2008 Olympics in Beijing. This week he alleged to the Guardian that he was forced out of the governing body as a result.

Before the London 2012 Games, it was alleged by the BBC that Azerbaijan, which loaned $10m to AIBA to underwrite a professional boxing series that has not been repaid, had effectively bought medals. A thorough AIBA investigation by senior officials rejected the allegations.

An AIBA spokesman this week told the Guardian: “Since June 2015, AIBA has undertaken major governance changes ensuring the long-term development of our sport according to the requirements of our business and sport partners and for the benefits of the AIBA worldwide community.”

But the senior officials who have spoken to the Guardian, who wish to remain anonymous for fear of reprisals against them or boxers from their country, have said the alleged corruption has become more sophisticated and more widespread since the London Games.

It is alleged corrupt officials, directed to score bouts in a certain way for a variety of reasons, are said to have initially relied on hand or head signals to manipulate judges at the end of each round so they knew from which corner to select the winner. But after being challenged at a number of major championships they are said to have changed their system, no longer relying on signals but meeting before major championships to decide on certain bouts.

“This is all being done very quietly,” said one senior source. “Some bouts are so bloody blatant it’s obvious. It sickens me to my stomach.”

A spokesman for AIBA said its mission was “to ensure the organisation of fair and transparent competitions”. He added: “It is not a matter for AIBA to interfere with the decision made by the judges under sporting rules.”

AIBA said there were approximately 300 judges rated three star and above who were used at elite competitions on a regular basis. It said its certification system ensured all referees and judges “have the highest levels of officiating and are in an optimal situation to perform accordingly”.

Sources alleged that at major championships the draw commission was also being directed as to which judges should officiate each bout.

AIBA says the draw is operated externally by Swiss Timing, their official scoring partner. There is no suggestion of any wrongdoing by Swiss Timing.

In some championships, sources claimed that around half of the bouts could be predetermined.

One boxing judge who was prepared to go on the record, Ireland’s Seamus Kelly, told the Guardian he had been asked to cheat at the Arab Games in Doha in 2011 by indicating who was winning during the fight.

He claimed a fellow judge had confided there were signals used under the new, so-called “10-9” scoring system at the European Championships in Minsk in 2013.

Kelly emailed Dr CK Wu, the sometimes controversial AIBA president, in April 2015 to raise the alarm.

Dr Wu replied: “AIBA will immediately investigate this case. We are zero tolerance for this kind of cheating judges.” Kelly heard nothing further and believes he has been sidelined.

“It was mental abuse. I wasn’t sleeping at all because of the way they made me feel. It put an X against my name. They were pushing me away,” Kelly said.

“People are afraid to speak out because if they are judges they will be sidelined and won’t get to go to the Olympics or other championships. If they are officials, they fear their country’s boxers will be targeted.”

AIBA said Kelly had been sent home from the youth world championships in 2014 for displaying aggressive behaviour towards one of his colleagues. Though he had since apologised, AIBA said due to the gravity of his actions and “personal issues” it was decided he was not fit to officiate.

“Mr Kelly’s allegations are baseless and damaging to our sport,” the spokesman said. “His correspondence to us over the last two years has been erratic, bordering on extortion at times, and once he understood he would not reintegrate into AIBA despite numerous emails, he has decided to launch false accusations against our organisation.” Kelly disputes AIBA’s version of events.

Another high-ranking source, who said he believed the manipulation was directed from the heart of the AIBA administration, claimed he witnessed scores being manipulated at major championships and says he is desperately concerned about what is likely to happen at the Rio Olympics.

Judges who make these claims are said to have been pressured to stay silent, while others are concerned they will be overlooked for championships and the Olympics if they speak out.

AIBA responded: “Boxing is a sport which triggers a lot of passion and sometimes people tend to behave as fans and not as boxing experts, which is prejudicial to the reputation of our sport. As for any Olympic Games, the expectations are very high and we can understand the importance of winning a medal.

“However, our role is to ensure a fair and transparent competition and that the thousands of spectators and millions of fans enjoy an amazing tournament with 13 great and undisputed gold medallists. We reiterate that, unless tangible proof is put forward, not just rumours, we cannot further comment on these allegations.”

The spokesman for the governing body added: “We will continue to use any means, including legal action, to protect our sport and its R&J [refereeing and judging] community whose integrity is constantly put into question despite their commitment and expertise. The future of AIBA boxing is bright and the organisation will not be deterred by subjective judgments made by discontented parties which do not reflect the AIBA community as a whole.”

Sources alleged scores are being manipulated to reward countries prepared to pay to host AIBA championships.

The international governing body has been accused of haemorrhaging cash since accepting a $10m loan from Azerbaijan to underwrite the expansion of WSB and AIBA Professional Boxing, a competition launched in 2014 to give professional boxers a “pathway” to qualify for Rio.

In a memo sent by ousted director general Ho Kim in June 2015 to AIBA executive committee members, seen by the Guardian, he underlines its “precarious financial situation” and the “rapidly expanding budget” of Wu’s presidential office in Taipei.

He also alleges Wu’s failed 2013 campaign to become the IOC president was covered out of AIBA funds, a claim AIBA denies on behalf of itself and Wu.

Of the Azerbaijan loan, Kim called on the executive committee to “properly understand what are Azerbaijan’s expectations as to the repayment of the loan or what other benefits are expected if the loan cannot be repaid”.

The AIBA spokesman said it was “in the process” of repaying the Azerbaijan loan but that it could not go into detail because of confidentiality clauses.

In a series of articles on the website Fight News, the Bulgarian journalist Ognian Georgiev last month made a series of detailed allegations about apparently suspect officiating at the Olympic qualifiers in the Venezuelan state of Vargas and the way in which judges were allocated to particular bouts.

Another senior boxing official claimed to the Guardian: “Simply because it costs so much to run a tournament, some countries are putting their hands out. You could interpret it in that way – look at Venezuela. Looking at the evidence, you have to wonder.”

The South American country is believed to have paid around $450,000 to host the Olympic qualifying tournament from which four of their six boxers who competed qualified for Rio.

Critics say countries who have hosted AIBA championships over recent years appear to have a correspondingly high proportion of boxers who qualify for the Games.

The AIBA spokesman responded: “The host city of Vargas was selected by a bidding process and a host fee was paid as is usually the case for all sports events organisation across the world.”

“We do not want to comment on rumours,” Wu told Georgiev. “Approximately 60 matches took place during the APB/WSB Olympic qualification tournament and all participants including the boxers praised the quality and fairness of the event.”

Terry Smith, a Welshman who sits on the AIBA executive committee and is the honorary life chairman of the referees and judges committee, insisted he had not heard any allegations of malpractice.

“I haven’t heard anyone speak like this. If it’s the case, the sooner someone alerts the president, the better. He is in the position to do something about it,” he told the Guardian. “I wish they’d confide a little bit internally. I would like them to challenge it where it takes place. It would be more helpful for us. You can be sitting right in the middle of something and not see it go on,” Smith added.

“These are the types of rumours we certainly don’t want with the Olympics coming up. With that many contests going on there is bound to be one that becomes controversial, the last thing you want to know is something like this throwing it into doubt.”