Two thirds (61%) of young people with cancer visited GPs with at least one of the most common cancer symptoms - yet for a third (28%) their doctors took no action, according to research.
A quarter (26%) visited GPs four times or more before their symptoms were taken seriously and they were referred to a specialist.
The research, conducted at Teenage Cancer Trust's 2012 conference for 300 young cancer patients, looked at the experiences of 13- to 24-year-olds when they first experienced symptoms of cancer.
Researchers said their findings highlight "the serious issue of delayed diagnosis" in the age group.
Misdiagnoses and feedback from GPs included:
- Infection or virus (15%)
- It's nothing/you're attention seeking (12%)
- Sports injury (10%)
- Stress, depression or psychosomatic (6%)
- "Take painkillers" (5%)
- Eating disorder (2%)
- "Come back in six months"
Three patients were told specifically "you don't have cancer".
Other misdiagnoses included telling patients they were suffering indigestion, vertigo and swine flu.
Professor Sir Mike Richards, national clinical director for cancer, said: "Early diagnosis is best achieved through the education of young people to increase their confidence in talking to doctors and helping everyone recognise the signs and symptoms of cancer in this age group.
"This is a major programme of work, and something which we are working closely with Teenage Cancer Trust to achieve."
More than a third (34%) of young cancer patients believe learning about cancer at school would have helped them identify their symptoms sooner.
The majority (59%) also want to see the signs and symptoms of cancer included in the national curriculum.
The findings come as Teenage Cancer Trust launches the first Teenage Cancer awareness week.
Running from today to May 4, the week will educate young people, parents, teachers and health professionals about the signs and symptoms of cancer in 13 to 24-year-olds.
The charity has also produced a schools pack to help teachers talk about cancer in the classroom.
Simon Davies, chief executive of Teenage Cancer Trust said: "We have been urging for cancer to be on the national curriculum for many years. "We're still waiting.
"That's why we've developed our own education team to help teachers tackle this difficult topic and created Teenage Cancer awareness week to help raise awareness of the five most common signs of cancer.
"Young people need GPs to take a 'three strikes' approach.
"If a young person presents with the same symptoms three times, GPs should automatically refer them for further investigation.
"The two week referral for suspected cancer is a major breakthrough but young people won't benefit until GPs think cancer quicker."
Five of the most common symptoms and warning signs in youngsters are unexplained and persistent:
- A lump, bump or swelling
- Extreme tiredness
- Significant weight loss
- Changes in a mole