Sheikh Salman bin Ibrahim Al Khalifa is yet to declare whether he is a candidate for the Fifa presidency

The controversy surrounding the possible candidacy of Sheikh Salman for the Fifa presidency has intensified with the International Trade Union Confederation expressing "deep concern" over the Asian football chief.

Salman has been accused by Bahraini human rights groups of complicity in the detention and torture of footballers and other athletes in a crackdown launched by the Arab kingdom's rulers following pro-democracy protests in 2011.

Campaigners have called for him to be prevented from standing for alleged "crimes against humanity".

Although Salman, who denies the claim against him, is yet to declare whether he is a candidate, the 49-year-old says he has been urged to stand "by a growing number of senior football administrators, Fifa members and personalities of public life".

He became favourite after the campaign of front-runner Michel Platini was thrown into chaos when the Uefa president was suspended over a £1.3m payment made to him by suspended Fifa president Sepp Blatter.

However, ITUC General Secretary Sharan Burrow said: "It's difficult to know how low Fifa politics can actually go. Football's governing body refused to investigate the allegations against Sheikh Salman from 2011, and it is inconceivable that someone who is facing such grave allegations of human rights violations could step into the void at the top of Fifa resulting from Swiss and US corruption investigations."

In 2013, three human rights organisations wrote to Fifa asking for Salman's nomination as head of the Asian Football Confederation (AFC) to be overturned. The Bahrain Institute for Rights and Democracy (BIRD) wrote to the then-chairman of Fifa's Ethics Committee Investigatory Chamber Michael Garcia, calling for an investigation into the allegations concerning Salman.

It alleged that "At least six footballers from the Bahrain national football team were arrested, defamed and tortured following their public identification and humiliation by authorities, including the Bahrain Football Association (BFA). More than 150 athletes, coaches and referees were jailed after a special committee, chaired by the former BFA president Sheikh Salman bin Ebrahim Al Khalifa, identified them from protest photos."

Garcia's response to BIRD was that the Chamber's jurisdiction was limited to investigating violations by football officials of applicable Code of Ethics provisions, and that the Chamber would therefore take no action.

In 2013, Sheikh Salman, who denies any wrongdoing, said: "I just have one question: You talk about allegations, but the question is, do you have the proof? Somebody talks about the government, I don't think this is our business in football. If anybody has the proof that the Bahrain Football Association has violated the statutes of Fifa or AFC then present it. Otherwise we move on."

Salman was also asked about the allegations in this interview with the BBC's Mani Djazmi external-link and insisted: "I follow the law and regulations".

Meanwhile, with Monday's deadline looming for candidates to officially announce their intention to run in the election (scheduled for 26 February 2016), global players' union Fifpro has published a check-list of criteria which it says all contenders for the job should meet, including "social wellbeing, fairness, democratic values and human rights."

In a statement, Fifpro said, "A clean break from the past is essential for Fifa to climb out of the toxic pit which continues to produce serious accusations of corrupt behaviour on almost a daily basis. At the same time, there is no doubt the present mayhem has left Fifa morally bankrupt.

"Fifpro has previously stated its dissatisfaction as having no confidence in Fifa's ability to reform from within. Individuals who have played a role in administering the game at the highest level are tarnished with the same brush, which is a sad reality for those who have displayed good intentions."

"Fifpro is afraid the current environment engulfing Fifa is not facilitating an effective election process and that it could produce an extremely harmful outcome."

All Presidential candidates will have to pass integrity checks before being allowed to take part in February's election.