Manchester United boss Sir Alex Ferguson has accused Jose Mourinho of going "below the belt" with his latest comments about Cristiano Ronaldo.
Ferguson and Mourinho previously had a cordial relationship
Last week the Chelsea manager called Ronaldo a "liar" after the winger disputed his claim that penalties are never awarded against United.
And at the weekend he continued the attack, saying: "Maybe it's about a difficult childhood, no education".
Ferguson responded: "It is really below the belt to bring class into it."
He added: "I don't know why he has done this. Maybe he is trying to unsettle the boy.
"Just because you come from a poor, working-class background does not mean to say you are not educated.
"What Ronaldo has are principles - that is why he has not responded to it. Other people are educated but have no principles."
Mourinho and Ronaldo both grew up in Portugal, but had very different upbringings.
While the Chelsea boss was raised in relative middle-class comfort in the city of Setubal, Ronaldo spent his childhood in the working-class district of Funchal in the Madeira Islands.
Manchester United could play Chelsea three times before the end of the season.
They meet in the Premiership at Stamford Bridge on Wednesday, will contest the FA Cup final at Wembley on 19 May and could clash in the Champions League final in Athens on 23 May.