Valentino Rossi has been stopped from testing with his new employers Yamaha before his contract with Honda expires.
Rossi aims to win Yamaha's first title for 12 years
Yamaha wanted the world champion to take part in an official three-day test at Sepang in Malaysia from 28 November.
But Honda said allowing that to happen before Rossi's deal runs out on 31 December would "concede advantages to our rivals".
Honda's team boss Carlo Fiorani added it "would be like giving a sixth card to an opponent in a game of poker".
The approach was made by Fortuna Yamaha team director Davide Brivio, who spoke to his Honda counterpart Fiorani last week.
"Despite our affection for Valentino Rossi and respect for Team Yamaha, we cannot allow Rossi to test the M1 machine before the 31 December," Fiorani told the championship's official website www.motogp.com.
"This is a game and you have to respect the rules. I think Davide Brivio was right to ask our permission, and I think we were right to turn him down."
Yamaha last had a world champion in 1992 when American Wayne Rainey won the last of his three titles.
Rossi, 24, announced after his victory in the season-ending Valencia race in October that he was quitting Honda and had been in negotiations with Yamaha.
He is expected to make his debut for his new team on board the Yamaha YZR-M1 in the season-opening South African Grand Prix on 18 April.