Officials were concerned the umpires would be exposed to excessive levels of radiation from television antennae and other sophisticated electrical and communications equipment on board the high-tech yachts.

"Because of the risk, it has been decided there will be no umpires on board," said America's Cup spokesman Bruno Trouble.

The umpires, who would have helped in the arbitration of possible protests during the first-to-five series, were to have been positioned on the transom of the yachts behind most of the communications equipment.

The yachts carry crews of 16 plus an unofficial observer as "17th man."

The decision to include on-board umpires was one of two issues decided at a mediation session led by America's Cup chief umpire Bryan Willis late on Thursday.

Willis said the decision was subsequently changed because umpires on Prada in particular would have had to stand within inches of a powerful microwave transmitter.

"It was decided that it was unfair to put people at risk," Willis said.

America's Cup holders team New Zealand had lodged an application to have the on-board umpires and a change to the system of penalties for infringements included in the sailing instructions for the regatta.

Under the current penalty system, yachts which infringe the racing rules must make a 270 degree penalty turn some time before they complete the race.

New Zealand wanted a change so that penalty turns must be completed immediately after an infringement was declared.

Prada opposed the change and it was decided during the mediation session that the existing penalty system would remain.

New Zealand is seeking to become the first successful non-U.S. America's Cup defender after it won the title off San Diego in 1995.