Some generic cancer treatments have soared in price by more than 1,000% in the last five years, affecting patient treatment, research shows.
Drugs used to treat breast cancer and leukaemia have become 10 times more expensive than they were five years ago, Dr Andrew Hill, a senior research fellow in the Department of Pharmacology and Therapeutics, the University of Liverpool, told the European Cancer Congress.
He said that "nobody knows these drugs are so cheap", estimating that the NHS is wasting £380 million a year across therapeutic areas through overcharging.
Paying these high prices puts undue strain on health systems, and the long-term result "could be negative consequences for disease progression and survival", the European Cancer Organisation said.
Dr Hill told the Amsterdam conference: "The idea with generic drugs is that the cost of research and development has already been paid, these drugs should be available at prices close to the cost of production."
He went on: "The budgets for the UK national health service and presumably across Europe elsewhere are suffering. The NHS is not approving some new cancer treatments just because the price is too high.
"National budgets to treat cancer are limited in most countries, and in most countries people simply won't be treated if the treatment prices are too high."
The researchers used three sources to compare the prices of cancer treatments between 2011 and 2016.
Twenty treatments showed rises of more than 100% in the last five years, and for two - Busulfan, used to treat leukaemia, and Tamoxifen, a breast cancer drug - costs per tablet or injection increased by more than 1,000%.
Busulfan went up from 21p to £2.61, an increase of 1,227%, while Tamoxifen increased from 10p to £1.21 - a 1,079% rise.
However, the percentage increases were in the tens of thousands when compared to Indian prices, he said.
In the UK, the prices of unbranded generic medicines are not controlled, with competition in the market relied upon to control prices.
The Government is introducing the Health Services Medical Supplies (Costs) Bill to regulate prices in the future.
Companies can also be referred to the Competition and Markets Authority (CMA), and face fines.
In December, the CMA said that Actavis had broken competition law by charging "excessive and unfair prices" after increasing the price of 10mg hydrocortisone tablets from 70p in April 2008 to £88 per pack by March 2016.
Earlier that month, Viagra-maker Pfizer was fined a record £84.2 million by the competition watchdog for its role in overcharging the NHS after the price of an epilepsy drug was hiked by up to 2,600% overnight.
Dr Hill went on: "We hope that, by explaining what we have found in the UK, other European countries will take note and protect themselves against these kinds of price rises.
"At a time when cancer patients are living longer and better lives due to effective treatments, this situation is particularly worrying."
A Department of Health spokeswoman said: "We are bringing in new legislation this year that will enable the Government to take action where competition in the market fails resulting in excessive price rises on unbranded medicines.
"We will continue to work closely with the pharmaceutical sector to make a success of these new measures, and all the money saved will be invested in the NHS to help provide the highest quality of care for patients."