Sir Alex Ferguson has increased the pressure on the referee Martin Atkinson for the Manchester derby by claiming there would be a House of Commons inquiry if Manchester United were awarded as many penalties as Manchester City.

Since the start of the 2010-11 season, City have been awarded 21 spot-kicks at the Etihad Stadium and in the past three games on home soil they have been given four – two against Aston Villa and one each in the draws with Real Madrid and Everton.

United visit City on Sunday and Ferguson said: "The number of penalty kicks they get – 21 in the last year or something like that. If we got that number there'd be an inquiry in the House of Commons. There'd be a protest."

United have actually been awarded more penalties, five, than City this season, only to miss three.

Ferguson's comments will add to the pressure on Atkinson during the derby, which the Manchester United manager accepts has risen markedly in profile since the days when he claimed Liverpool were United's true rivals.

"Liverpool and United games over the last 25 years have been unbelievable," he said. "They've always been the most important games. It's shifted because at this moment Liverpool aren't challenging for the league like City. They are our biggest threat and we're their biggest threat, so that's changed. Their fortunes changed the minute Sheikh Mansour took over.

"We knew the minute that happened it was going to be a different ball game altogether. But, as we have in the past, we have to accept their challenge like we did when Chelsea came along and when Arsenal overtook Liverpool in the early 90s. We have to do it again."

Ferguson is still to discover whether Tom Cleverley will be fit after limping out of Wednesday's Champions League defeat to Cluj with a calf injury.

Antonio Valencia and Nani have been ruled out, while faint hopes that Shinji Kagawa and captain Nemanja Vidic could be involved appear to have faded.

"I thought Kagawa may have done enough to make the bench on Wednesday but no, he's well short," Ferguson said.

"I don't think he'll be available until the Sunderland game next week. Vidic is certainly doing well in training. It suits his own confidence to play but I think the game on Sunday is a bit soon for him."