FIFA has imposed tighter transparency rules on World Cup bidders when they seek meetings with voters who will choose the 2018 and 2022 hosts.
World soccer's governing body said Thursday that the nine bid teams must explain in writing why they need access to the 24 Executive Committee members or voters' national associations.
FIFA told bidders in a statement that they are forbidden from trying to influence voters by ''offering benefits for specific behavior.''
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.''
FIFA's warning comes days after the World Cup ended, and much of the organization's focus shifts toward a blockbuster vote on Dec. 2 to award back-to-back World Cup hosting rights.
The United States has retained its entry for the 2018 World Cup, which is expected to be awarded to one of four European bids: England, Russia, and the joint bids of Spain-Portugal and Netherlands-Belgium.
The U.S. is competing with Japan, Qatar, South Korea and Australia to host the 2022 World Cup.
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 brings FIFA 95 percent of its total income, and is budgeted to earn at least $3.8 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 criticize 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 bid teams to register any planned attempt to contact 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 letter. ''Any violation of this principle will lead to an investigation ... with possible sanctions in accordance with the FIFA code of ethics.''
The ethics panel, chaired by former Switzerland international forward Claudio Sulser, is finalizing its investigation into the behavior 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 soccer projects.