FIFA has imposed tighter transparency rules on World Cup bidders in the wake of its investigation into the ethics of Australia's bid for the 2022 tournament.

World football's governing body has told the nine bid teams they must write explaining why they need access to the 24 members of FIFA's Executive Committee - or those members' national associations - who will choose the host nations in December.

"We would like to inform you about a new policy we ask you to strictly respect," FIFA told the bidders in a statement released Thursday.

FIFA reminded candidates they are forbidden from trying to influence voters by "offering benefits for specific behaviour."

The 24 executive voters noted that "various" bid teams approached them "either formally or informally" during the World Cup in South Africa.

FIFA said it wanted to "better monitor these contacts and also preserve the independence of members in the bidding procedure" to help ensure a clean fight for the rights to host the tournament in 2018 and 2022.

FIFA's warning is published days after the 2010 World Cup ended, and much of the organisation's focus shifts toward a blockbuster vote on December 2 to award back-to-back World Cup hosting rights.

It also comes as the ethics panel, chaired by former Switzerland international forward Claudio Sulser, finalises its investigation into the behaviour of Australia's bid team.

The probe was launched last month after Australian newspapers reported that bid leaders offered jewelry to FIFA voters and their wives, and offered to pay travel expenses for football projects.

Next week, a FIFA team of technical experts will begin inspection visits of the nine bids to assess their ability to host the tournament.

The World Cup gets FIFA 95 percent of its total income, and is budgeted to earn at least $US3.8 billion ($A4.3 billion) in commercial revenue for the 2014 tournament, which is being staged in Brazil.

FIFA bid rules say candidates can give voters gifts of merely symbolic value which must not influence the bidding process.

Candidates also cannot criticise rival bids or make deals to trade support on the FIFA ruling executive, with eight of the 24 members representing countries in the bidding races.

The new transparency rule requires all nine bid teams to register any planned attempt to contact those voters or their national association.

All contacts "shall be reported in advance and in writing to the secretariat to the FIFA Ethics Committee," the governing body's secretary general, Jerome Valcke, said in the circular letter.

"Any violation of this principle will lead to an investigation ... with possible sanctions in accordance with the FIFA code of ethics."

Australia is competing with Japan, Qatar, South Korea and the United States to host the 2022 World Cup.

The US has retained its entry for the 2018 contest, which is expected to be awarded to one of four European bids: England, Russia, and the joint bids of Spain-Portugal and Netherlands-Belgium.

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