World Cup soccer sponsors Adidas and Sony voiced concerns over allegations that bribes were paid to secure the 2022 tournament for Qatar, an issue that is overshadowing this week's kickoff in Brazil.

With its four yearly showpiece event only four days away, soccer's governing body FIFA is on the defensive and is conducting an internal investigation into the decisions to hold the 2018 World Cup in Russia and the 2022 Cup in Qatar.

Qatar's bid in particular has attracted controversy from the outset because of the extreme summer heat during the months when the Cup is played and the tiny country's lack of domestic soccer tradition. If it goes ahead, the tournament is expected to be switched to a date later in the year, creating scheduling headaches for broadcasters and European club soccer clubs.

The signs of unease from some of FIFA's paymasters will raise pressure on the body, led by its Swiss president Sepp Blatter, to take a firm line on Qatar and underlying concerns about how it is run.

"The negative tenor of the public debate around FIFA at the moment is neither good for football nor for FIFA and its partners," said German sportswear company Adidas, which has signed up as FIFA sponsor until 2030.

Japanese consumer goods company Sony said it expected the allegations to be "investigated appropriately".

Britain's Sunday Times newspaper has printed what it says are leaked documents showing bribes were paid to secure the event for Qatar, which Qatar denies.

Former US prosecutor Michael Garcia, leading FIFA's internal investigation, is due to report in July, around a week after this year's World Cup final.

It is unusual for sponsors to say anything publicly on such a sensitive issue and the comments from Adidas and Sony reflect concern over the knock-on effects on their image.

"This underlines that companies need to make sure that any high profile association enhances their reputation rather than damages it," said Andy Sutherden, Global Head of Sports Marketing & Sponsorship at communications firm H+K Strategies.

FIFA earned almost $1.4 billion last year, including more than $600 million from the sale of broadcasting rights and more than $400 million from sponsors and other marketing partners.

Sony and Adidas are among six main FIFA sponsors who collectively paid around $180 million last year. Sony's sponsorship agreement, which also included the 2010 World Cup, expires this year, giving it particular leverage as it negotiates a new deal.

The Sunday Times printed new accusations on Sunday, just four days before the 2014 tournament kicks-off in Brazil, alleging that then-Asian football chief Mohamed Bin Hammam, a Qatari, had brokered meetings between Qatari officials and governments to discuss bilateral trade deals.

Qatar denies Bin Hammam was connected to its bid for the Cup. Bin Hammam has not commented. FIFA has already banned Bin Hammam for life from soccer over accusations he paid bribes to win votes for a bid to become FIFA president. That ban was overturned but another was imposed for conflicts of interest.

FIFA is now facing calls to strip Qatar of the World Cup should Garcia's investigation prove that it bought the votes needed to host the tournament.

The US, Australia, Japan and South Korea were the rivals who lost out to Qatar in the 2010 vote and are all safer choices as former hosts of major sporting events and developed markets for consumer brands.

US broadcaster Fox, which paid an estimated $425 million for rights to the 2018 and 2022 World Cups, was already unhappy with plans to switch the dates of the Qatar tournament to later in the calendar year when it would clash with the NFL American football programme.

Both fans and sponsors would be likely to cheer a switch to a different venue, although Qatar would be certain to mount a legal challenge to keep the Cup.

"None of the sponsors would want it to be held in Qatar," said David Peters, managing director of marketing company Dentsu Aegis Network Sport & Entertainment.

"FIFA haven't given a great deal of consideration to sponsors. The sport is so big, they are less beholden than other sports," he added.-Reuters