The newly knighted ex-Beatle tried to head off criticism after it emerged that he had prevented Lily Evans, 60, selling the paper on which he had scribbled the words for "With A Little Help From My Friends".
She had hoped to raise pounds 60,000 for her old age by selling the paper at Sotheby's, she told a BBC1 Watchdog investigation screened last night.
Her husband Mal had been the Beatles' road manager for many years and had been particularly close to McCartney. But he died in a shooting accident in Los Angeles 21 years ago, leaving her without a pension.
The Beatles sent her pounds 5,000 at the time of his death which had been "most helpful", but she had worked as a secretary to support herself since and she could not understand why Sir Paul was stopping her gaining a nest- egg for her final years.
"I didn't know why he would want to do that. It wouldn't be for the money and he lets other people sell, so I don't know why he would want to stop me," she told the programme.
"If my husband had remained in his post office job I would have been better looked after."
Her son backed her, saying: "I think of everything dad did for him. He'd be on 24 hours' notice and he'd do anything for Paul - he loved the guy. To do this to my mum now, I just don't think it is right. I don't think he can have much of a conscience."
But in an angry statement Sir Paul said: "The programme is trying to make the Beatles out to be widow-beaters - nothing could be further from the truth.
"I would like to meet Mrs Evans and discuss this and come to some arrangement to see that she is taken care of and that the lyrics are returned. They were never Mal's lyrics and therefore any relative of Mal's such as Mrs Evans does not have the right of ownership to these lyrics.
"To show how ridiculous this whole memorabilia market has become, there is someone in the US who owns my own birth certificate. How people can feel that that is right is beyond my comprehension.
"I am surprised that Watchdog is doing this report. I thought Watchdog was normally on the side of people who have been ripped off - not on the side of people who are doing the ripping off."
He added: "I don't wish to cause any trouble for Mrs Evans or for her children, whom I remember fondly, but I do feel strongly that these original manuscripts should be returned to their rightful owners."
Sir Paul's lawyers have taken out an injunction stopping Mrs Evans from selling the paper until the case over ownership comes to court, which could take up to a year.
His spokesman, Geoff Baker, said Sir Paul had twice offered to help Mrs Evans if she was in hardship, but she had not taken him up on it. He was prepared to make her a "substantial" personal donation.