Jose Mourinho has ratcheted up Chelsea's rivalry with Manchester United by warning that Cristiano Ronaldo would be a liar if he failed to agree with the Blues' coach in a controversy over penalties.

The trouble started last weekend when Mourinho claimed on British television there was a new rule in English football, that penalties would not be given against United or for Chelsea.

In response, United winger Ronaldo was quoted by the Portuguese media this week as saying: "I don't want to be dragged into Mourinho's complaints about referees, everyone knows how Mourinho is.

"He has always something to say and cannot admit his failures."

The barb drew a sharp reaction from his Portuguese compatriot Mourinho, who had made his views clear on alleged fouls by United players in the penalty area against Sheffield United and Middlesbrough.

"If Ronaldo says these fouls never took place, he is lying," Mourinho was quoted as saying by Diario de Noticias.

"And if he lies he will never achieve the dimension he wants to have (as a player)."

United's Portuguese coach Carlos Queiroz sidestepped the penalty controversy but was clearly unimpressed by Mourinho's attitude.

"Mourinho always tries to call attention to issues that are convenient for him," he said.

"It is his tactic. We are different because we have the humility to recognise merit in the adversaries."

The two clubs are at loggerheads on two fronts -- and potentially a third in a nail-biting end to the season.

United are three points ahead of Chelsea in the Premier League with four games left to play -- with one of those being a showdown between the two sides at Stamford Bridge on May 9.

They will then meet 10 days later at the new Wembley stadium for the FA Cup final and could also face each other four days after that in the Champions League final in Athens.

United lead AC Milan 3-2 after the first leg of their semi-final, while Chelsea are 1-0 up over Liverpool before next week's decisive second leg matches at San Siro and Anfield.

(Reporting by Henrique Almeida in Lisbon; additional writing by Trevor Huggins in London)