A £200m-a-year fund for life-enhancing cancer drugs is to continue until 2016, the prime minister has announced.
The Cancer Drugs Fund (CDF) was set up in 2011 to help patients in England access certain drugs before they get approval for widespread NHS use.
The scheme was due to end next year, but David Cameron has pledged £400m to keep it running.
Cancer charities have welcomed the move, but Labour accused the prime minister of "letting down" patients.
Head of policy at Cancer Research, Sara Osborne, praised the valuable role the fund would play in battling the disease and highlighted the tens of thousands of people who received treatment because of it.
She said: "There's about 30 drugs that are available on the Cancer Drugs Fund, and over the last three years about 34,000 patients have had treatment that they would not have otherwise had, had the fund not existed."
The aim of the CDF was to make it easier for medics to prescribe treatments even if they have not yet been approved by the National Institute for Health and Care Excellence (Nice).
The scheme was set to run until 2014 and campaigners raised concerns about where patients would turn for help when the funding ceased.
Prime Minister David Cameron said the CDF had been a "massive success" and added that should he be re-elected he would recommend that it be continued beyond 2016.
"People have lived longer and in some cases it has saved people's lives," he said.
Health Secretary Jeremy Hunt said the government had made an exception for cancer because they considered it "the number one killer"
"And we do think that we had a particular problem with a lack of access to these drugs," he added.
Andrew Wilson, chief executive of the Rarer Cancers Foundation, said: "This is a compassionate, common sense announcement which will be warmly welcomed by many thousands of cancer patients."
The Rarer Cancers Foundation estimates that 16,500 extra patients will benefit each year as a result of the extension.
Mr Cameron said the government would also be partnering with Cancer Research UK to conduct new research into the effectiveness of cancer drugs.
"It is only because we have protected health spending that we can afford these life saving treatments," he added.
But another charity, Target Ovarian Cancer, said while the news was "positive", it warned it was "just a stop-gap".
It highlighted the case of Jenny Bogle who failed to meet CDF criteria for Avastin, a drug shown to delay recurrence.
Ms Bogle said: "I have a wealthy friend and an oncologist who are determined to keep me going for as long as possible so I was able to access Avastin privately in the end.
"I'm lucky to be here but it's just not fair.
"It shouldn't be a matter of luck. If I were living in a different postcode, were vulnerable or less pushy or didn't have the support, I would have probably died years ago."
BBC health correspondent Nick Triggle said the move also raised questions about the introduction of a new way of assessing drugs that had been expected to start next year.
Next year was meant to mark the start of value-based pricing, a system proposed by previous health secretary Andrew Lansley to ensure there is a closer link between the price the NHS pays and the value that a medicine offers.
There will now be no full blown overhaul although drug advisory body NICE will be looking to make some changes to improve the assessment process from January.
Shadow health minister Liz Kendall highlighted the fact that expert cancer networks - set up to improve access to high quality services - were scrapped during the reorganisation of the NHS earlier this year.
She added: "David Cameron should also stand up to the tobacco lobby rather than caving in to them over standardised cigarette packaging, which experts say would be a powerful weapon in the long-term fight against cancer."
Alongside plans to extend the fund, Mr Cameron also announced that Genomics England - a government-owned organisation tasked with mapping the DNA of 100,000 patients with cancer and rare diseases - will begin a partnership with Cancer Research UK.