The poor are not benefiting as much as the rich from improvements in cancer survival, experts have revealed.
Better off patients may feel more able to demand the best care
An analysis for Cancer Research UK and the Office of National Statistics showed record numbers of patients are surviving cancer.
But the data for England and Wales showed the gap in survival rates between rich and poor has widened.
Experts suggest the wealthy may more willing to push doctors to provide the best possible care.
They said the findings showed more work was needed to ensure all patients benefited from improvements in cancer care.
Equal access to treatment was one of the key aims of the 10-year NHS Cancer Plan, launched in 2000.
Researchers at the London School of Hygiene and Tropical Medicine looked at 2.2million adults diagnosed with cancer in 1986-1990, 1991-1995, and 1996-1999.
They found survival improved for 15 of the 16 cancers examined among men, and for 13 of the 17 cancers examined among women over the whole period studied.
But researchers found the gap in survival between rich and poor increased for 12 of the 16 cancers in men and for nine of the 17 cancers in women.
For example, survival for rectal cancer in 1996 to 1999 was 9.4% higher for the richest patients than the poorest patients in men in that period, and 8.3% higher in women.
Poorer patients have always been less likely to survive cancer, but the gap widened over the period studied.
It grew by around 2.5% for both men and women between 1986-90 and 1991-1995, and again by 1996-1999.
There were also significant differences in survival rates in cancers of the oesophagus, colon, rectum, larynx and prostate in men and the colon and bone marrow cancer in women.
Professor Michel Coleman, who led the research, said: "Our analyses show that cancer survival is likely to continue improving for people being diagnosed with the disease now.
"But while this is great news we need to take action to ensure everyone benefits equally from advances in the detection and treatment of the disease."
'High quality services'
Professor John Northover, director of the Cancer Research UK Colorectal Cancer Unit, told BBC News Online many factors could influence the disparity seen between rich and poor: "People with problems such as losing their job may not give looking after their own health the attention it deserves.
"And it may be that, as a result of that, their disease may be further down the track when they seek help."
He added that patients from more deprived areas may also not have had access to developments in early diagnosis and chemo or radiotherapies, as those from wealthier areas.
And he said: "People who are relatively better off may feel more confident about pushing clinicians for information, and for treatments they have seen on the Internet.
"But poorer people may feel more intimidated by the hospital environment, by doctors, and be less likely to question their treatment."
Professor Northover said recent initiatives such as national targets for seeing suspected cancer patients should help narrow the gap.
Professor Alex Markham, chief executive of Cancer Research UK, said: "These figures highlight the importance and relevance of the government's Cancer Plan and the scale of the task ahead."
Health Secretary John Reid said: "This new study, as Cancer Research UK acknowledges, covers patients who were diagnosed before the Cancer Plan was published and does not reflect recent efforts to tackle inequalities."
He added: "The plan recognised that the poor were more likely to get cancer than the rich and their chances of survival were lower so it set out strategies to ensure that all patients have access to uniformly high quality services and the most effective drugs."
The research is published in the British Journal of Cancer.