The success of the trial - led by The Institute of Cancer Research (ICR) and the University of Birmingham - could mean fewer patients need their bladder removed and provides a viable alternative for frailer patients who are too weak for surgery.
The results are published in the New England Journal of Medicine on Wednesday.
Study co-leader Professor Nick James from the University of Birmingham, said: "Bladder cancer is largely a disease of older people and is also linked to smoking, meaning patients are often in relatively poor general health when diagnosed.
"Removing the bladder is still one of the most effective ways of treating invasive cancer that has spread to the muscle of the bladder. But this can be very distressing and we know patients are often too frail for such radical surgery.
"The alternative is to give radiotherapy, but around a third of these patients will go on to relapse with invasive disease and will need their bladder removed anyway. So these results really provide a lifeline for those too old or weak for surgery and mean that, in future, fewer patients will need their bladder removed."
360 patients from around the UK were included in the study. Around half were given two commonly used chemotherapy drugs - fluorouracil and mitomycin C - in addition to the radiotherapy treatment.
33% of patients receiving chemotherapy in addition to radiotherapy (known as chemoradiotherapy) had a relapse within their bladder or surrounding tissues within two years, compared to 46% of those who had radiotherapy alone.
Of those who received chemoradiotherapy, around one in five developed invasive cancer - the most serious form of the disease - compared to around one in three among those who had radiotherapy alone.
Early results also showed that combining radiotherapy with chemotherapy may improve survival, with 48% of patients still alive after five years, compared to 35% of those who had radiotherapy alone, although larger studies are needed to confirm this.
Study co-leader Dr Robert Huddart from the ICR and The Royal Marsden NHS Foundation Trust, added: "Removing the bladder is a major operation with implications for the rest of the patient's life. We found that adding chemotherapy to radiotherapy successfully lowered the risk of relapse sufficiently to make it a real option for patients who don't wish to have radical surgery and lose their bladder. Importantly, these improvements have been achieved with drugs that are cheap and widely available, and carry few additional side-effects over radiotherapy alone."
Wendy Powell is 49-years-old and lives in Birmingham. She agreed to take part in an earlier pilot study after tests revealed a tumour in her bladder "the size of a lemon".
She said: "When I was told I had bladder cancer it was a shock at first. They went through all the options for treatment and when they mentioned the trial I decided to give it a try. I did have side-effects and it has all been a hard journey, but in the end it's been worth it because here I am thirteen years later. I'm back to normal now and it's thanks to clinical trials that more people are surviving like me. I'm all for trials. It's really good they're coming up with new ways of treating the disease."
Each year in the UK around 10,400 people are diagnosed with bladder cancer. It is responsible for around 5,000 deaths per year. Bladder cancer is most common in older people, and is the seventh most common cancer in the UK.
Kate Law, director of clinical research at Cancer Research UK, said: "These findings provide a new 'gold standard' of treatment that will be particularly important for elderly patients, because surgery to remove the bladder in this age group can have a severe impact on quality of life.
"Survival rates for bladder cancer have been increasing in recent years, with around half of patients now surviving 10 years or more, compared to around a third in the 1970s. But bladder cancer is largely a disease of older people and, with an ever ageing population, it's essential that there are alternative treatments suitable for this age group."
Source: Cancer Research UK
