Piquet's famous father and namesake is quoted by the Spanish newspaper Diario Sport as suggesting the Spanish racer "knew everything" about the deliberate crash in Singapore last year.
The crash triggered the safety car and activated Alonso's unusual low-fuel strategy. The 27-year-old went on to win the race but at Monza last weekend he denied he knew about the allegations until news of the scandal broke.
FIA president Max Mosley also said at Monza that there is no information that Alonso knew about the deliberate crash.
But Piquet Snr told Sport: "Fernando knew everything. He couldn't ignore it.
"If you are 15th on the grid at a street circuit, there is no point going out with no fuel. At most you will pass three cars and after your last stop you stay where you are. It's a senseless strategy," the Brazilian former triple world champion added.
Bernie Ecclestone, the F1 chief executive and a close ally of Renault team boss Flavio Briatore, was also quoted by the Spanish newspaper.
The 78-year-old reportedly said: "Nelson told me that he is going to do everything he can to destroy Briatore."