One of the world's greatest opera stars has made an impassioned plea for Britain to stop blocking the flow of young singers into opera houses so that the top quality talents of the future can flower.

For almost half a century Dame Kiri Te Kanawa has sung for princes, queens and commoners with a warmth and technical precision that has marked her out. And she is still singing to great acclaim in occasional concert performances after celebrating her 70th birthday this spring.

But the lyric soprano from New Zealand worries for the future. "We are seeing a lot of disposable singers these days," Dame Kiri told the Observer this weekend before giving a rare, extended solo performance at a new music festival in Mallorca.

"These new singers last about five years and then the next tenor comes through. I hope I am not speaking out of turn here, but television talent shows have a lot to answer for: Britain's Got Talent and The X Factor. If you win, it is 'zing', there you are, and for five minutes you are famous."

The opera diva became a household name in Britain when she sang at the wedding of Prince Charles and Diana in 1981, a performance heard by 600 million people. Her remarkable singing voice was enjoyed again by television viewers all over the world last year when she appeared in the role of the Australian singing star Dame Nellie Melba in an episode of ITV's drama series Downton Abbey.

Dame Kiri, who grew up in New Zealand but now spends much of her time in her London home, is now calling for leading British opera companies to allow young singing talent from outside Europe to come into the country for longer-term training, as she once did, so that performers can build up the endurance and technique they need to survive on the international opera circuit. She believes that many gifted young people from outside Europe are failing to access the best teachers, while the ill-prepared winners of television talent shows hog the limelight.

"I get criticised for attacking talent shows like The X Factor, but people often ask me when the singers they have seen on television are going to make it on the opera stage. The truth is they have got to learn how to sing properly first, but it is very hard for them to get the right training and experience," she said.

According to the soprano, an emphasis on good looks and a beguiling personal story, rather than on singing skill, has been promoted not just by popular talent shows but by a growing reliance on televised opera productions and the live opera house productions that are relayed to cinemas.

"They sometimes put slimmer singers on TV for productions. They may look good on telly and somehow get through it, but they can't really sing, and then on stage they have to use a bigger, better singer. I have seen that happen," she said.

Dame Kiri is also sceptical about the impact that opera screenings in cinemas are having on broadening the appeal of opera: "I don't know if young people are going to see opera in the cinemas, and I don't think it helps the operatic world generally. I don't think it helps sell seats in the opera houses, and it creates homogenised-sounding singing. It all sounds the same."

Although Dame Kiri stopped making regular opera appearances a decade ago, her top-flight singing career has lasted more than 45 years. She came to England when she was 21 and was singing at Covent Garden at the age of 27. Originally a mezzo-soprano, she specialised in singing Mozart, Strauss, Verdi and Puccini. "Nowadays I can't see any new singers lasting even 15 years," she said.

Much of her time is spent working with students through her Kiri Te Kanawa Foundation or in fundraising for students and judging singing competitions, such as Cardiff Young Singer of the World. She currently supports eight singing students, she said, but remains concerned that the British visa system stands in the way of many of the international pupils with most potential.

"When I came over there was an open door. But now a foreign singer can come into schools for a year or so, but then, more often than not, if they want to carry on they have to reapply. So what happens is they have to go back after their time runs out and sit there in New Zealand, for example, for almost a year. I talk to opera companies in London about it, and they don't like it when I say it. But these singers have got to get the visas or they will never learn."

One extra place a year for a student in London, she believes, would be enough to allow the cream of foreign talent from outside Europe to make its mark.

"The thing is, they are not all going to make it, so what I want is just one place a year for a person who is really going to last. I have been working with one wonderful student, a New Zealander called Phillip Rhodes, for five years, and he is just coming through now, and that has been a huge struggle." Rhodes sang in Benjamin Britten's opera Peter Grimes with Opera North this year.

She understands, she said, that Britain is over-populated, but she feels that places for devoted students who are prepared to invest time and thousands of pounds to learn their craft would be money well spent.

"They are so dedicated. There is total and utter focus and devotion," she said. "They will do anything. And I also have to complain about the fact that, if they are allowed to stay, they are not allowed to sing in public or to be paid for singing. Imagine if a young foreign footballer came in and he was not allowed to play football? Tell me who is making up these rules?"

Young singers are inspired by the opera stars they see in their youth, Dame Kiri believes. "My students come to opera because of me. And I did it because of Joan Sutherland, while others did it because of hearing José Carreras or Plácido Domingo or Pavarotti. All of these young singers follow their own Pied Piper.

"It is just so rewarding for me to teach them. I can't tell you how gorgeous it is. It is a pleasure to work with them because they are so devoted to it."

There is little chance of building up a truly elite tradition of singing outside Europe, she argues, because the opera houses in New Zealand and Australia mount only a few performances a year and are bedevilled with cash crises. "There are great, dedicated young singers coming up all over the world, and yet there is no space. No jobs, because there are only so many places at the top table," she said.

Her concert this weekend at the Barcelo Formentor Sunset Classics Festival will be followed by a fundraising concert in Wales later in the year, but Dame Kiri plans a quieter time in 2015. She stressed, however, that she is not tempted by retirement."No, no, no. Not yet," she said.

The soprano added that during her long career she has always tried to reach for perfection but it has never yet happened. "Time is running out, but I still try. I think I have achieved a lot, but it is never enough. The next concert is always the best one."