NHS reforms and squeezed budgets risk stalling recent advances in fighting cancer in England, a spending watchdog warned in a report highlighting the starkly lower survival chances of poorer and older patients.
The National Audit Office (NAO) said action was needed to reduce the almost 20,000 deaths a year that would be avoided if those from deprived areas fared as well as the better off, but said there were now "fewer dedicated resources" to improving services.
Its report also found "important gaps" in data were hampering the ability to choose the best treatments for patients and ensure they represented good value for taxpayers' money - in the same week a cost-cutting review ended funding for 25 drugs.
Those changes were made to the Cancer Drugs Fund (CDF) - which provides exceptional access to expensive treatments - but the NAO suggested it was "difficult to evaluate in a meaningful way" the benefit of the £733 million already spent on the scheme.
Health chiefs face a grilling by MPs next week over what the chair of the Commons Public Accounts Committee (PAC) said remained "shocking disparities" with other European countries, "astounding" differences within the UK and consistently missed targets on time between referral and treatment.
The report - released with warnings of extreme pressures on the NHS at the centre of political debate ahead of May's general election - welcomed some notable improvements since it last investigated cancer care in 2010.
Average survival rates after both one and five years are up by four percentage points, to 69% and 49%, awareness campaigns had helped significantly increase the numbers seeking earlier help and 89% still rated their care excellent or very good.
However, it said there was "considerable scope for further progress" with one in five still only being diagnosed in an emergency situation - leaving them twice as likely to die within 12 months - and a target of having 85% treated within 62 days of a GP referral not met since the end of 2013.
Overall survival rates remain 10% below the European average, a key target under the most recent cancer strategy, with the elderly in the UK suffering notably poorer outcomes than their equivalents - although children tend to do as well as elsewhere.
And with charities and some medical professionals warning the system is operating at its limit, the NAO concluded: "There is a risk that the significant progress that has been made in improving cancer services and information will stall as a result of recent changes and pressure on resources."
The Government's shake-up of commissioning had resulted in more complex referral arrangements that were still "bedding in", there was "confusion" about responsibilities and accountabilities and it was "unclear" how NHS England was monitoring performance or whether any one body had national oversight.
Since 2010, there were also "fewer dedicated resources" to support improvement in cancer services, the NAO found - pointing to the 2013 disbandment of the National Cancer Action Team and related regional bodies.
On the issue of informed decision making, the NAO raised concerns that "commissioners have only limited assurance that they are buying services that offer the best outcomes for patients or that they are achieving value for money".
There are also concerns about whether the health system is in a position to "exploit effectively the wealth of data that are available".
As an example, the report noted that, in 2013/14, half of relevant CDF patients were not recorded in chemotherapy figures - though it said that issue had since been addressed.
Despite significant progress, "important gaps in cancer data remain - particularly in the availability of robust cost data", it noted. The report added that recent controversy over patient privacy may also have contributed to important information being less readily available.
Specifically addressing the CDF, set up as a temporary measure by David Cameron in 2010, the report concluded: "Data gaps also make it difficult to evaluate in a meaningful way the £733 million spent."
NHS England has announced that the budget for the Cancer Drugs Fund (CDF) will rise to £340 million in April from £280 million.
But a review carried out by doctors, pharmacists and patients' representatives has concluded that 25 of the 84 treatments currently offered by the supplementary fund should be dropped in a bid to prevent a significant overspend.
Labour MP Margaret Hodge, who chairs the PAC - which asked the NAO to investigate and has called in NHS chiefs for a hearing on the issue next week, welcomed improvements but said she was "deeply disappointed that cancer survival rates in England remain poor compared with the best European countries".
She added: "There are also unacceptable inequalities within England that must be tackled.
"I'm astounded that, when it comes to getting and surviving cancer, the difference between the most and least deprived is so stark that if mortality rates were the same for the richest and poorest there would be almost 20,000 fewer deaths from cancer each year.
"Big gaps in data on the cost and efficiency of cancer care treatments mean that commissioners still cannot make informed choices about how to spend finite resources wisely.
"It ought to have set alarm bells ringing when, between January and March 2014, the NHS failed to meet its standard to treat 85% of cancer patients following an urgent referral from their GP within 62 days - the first time this waiting time standard was missed since it was introduced in 2009-10.
"This target was again not met for the next two quarters of 2014. Between July and September, 5,500 people had to wait more than 62 days for their treatment to start.
"I expect to hear how the Department of Health, NHS England, Public Health England and NICE will close the gap in outcomes both within England and with those better performing European countries, when they appear before the Committee on 21 January."