It's a controversial question which has long divided the world's leading scientists: can stress cause cancer?

For years, doctors treating patients have often observed that severe life events – bereavements, divorce and serious trauma – often occur in the years immediately before the onset of the disease.

Yet the actual evidence is far from conclusive. Even Cancer Research UK (CRUK) says the links are not only unproven but may be little more than a myth.

But with stress levels on the rise, and the number of people with cancer expected to increase by one third to four million by 2030, some cancer experts now say emerging evidence on the impact of stress on the body means it would 'not be surprising' if there was a link between the two.

Professor Melanie Flint from the University of Brighton, who studies the impact of stress hormones on cancer, says: 'There are a lot of advances in this field and I don't think we can rule out the contribution of stress to cancer.

'My view is that it does contribute, both to the initiation of cancer in the first place and the spread of cancer once you have it – but it's a contributory factor, not necessarily a direct cause.'

Some studies which have followed large populations over time appear to back this up. 

One study of 10,000 women in Finland, followed for 15 years, found those who had suffered bereavements were more likely to develop breast cancer within five years.

Exposure to workplace stress has been associated with a greater risk of prostate cancer in men under 65 and, although it was a weaker link, to breast cancer in women.

Other similar studies, however – including reviews of the best quality evidence – fail to show any association at all.

Part of the problem, according to Professor Trevor Graham, director of the Centre for Evolution and Cancer at The Institute of Cancer Research, is that stress often goes hand in hand with other behaviours which also increase the risk of cancer.

'A stressful life may be inter-related with lots of other risk factors for cancer, such as smoking, drinking too much alcohol, inactivity and an unhealthy diet, so it's hard to tease the causative factors apart,' he says.

What we know is that stress does cause a cascade of effects on the body – particularly if it is chronic.

Professor Flint says: 'Stress causes the release of the stress hormone cortisol.'

'Cortisol binds to receptors which are on every cell, and this regulates various other processes, including inflammation.

'Stress and cortisol can also suppress the immune system.'

Scientists have also tried to mimic the effect of stress on cells in the lab, which appears to show it damages DNA and causes changes to cells which, if allowed to replicate, can turn into cancer.

But because stress is hard to model, this might not be a good indication of what is really be going on in the human body, points out Prof Graham.

It's also likely that most people can repair this damage naturally. 

But there is a notable exception: those with genetic mutations which already put them at higher risk of cancer – such as those with the 'Angelina Jolie' genes BRCA1 and BRCA2, named after the Hollywood actress who inherited them from her late mother, who died of the disease – may be less able to do so, Professor Flint says.

'If someone has a cancer mutation which affects how well they can repair DNA, such as BRCA, they may be more vulnerable to the effects of stress,' she says. 

'We can't say that if you're highly stressed, and you have a cancer mutation, that you'll definitely get cancer. We still don't quite understand all of the underlying mechanisms.

'But we may need to bear it in mind as a possible additional risk for some people and stress may need to be managed.'

There is also some limited evidence, she adds, that stress may indirectly increase the risk of cancer developing by causing viruses, such as the human papillomavirus (HPV) linked to cervical cancer, to flare up.

Where the evidence is strongest, however, it suggests that stress may well have a role in causing cancer to spread after it has already developed.

Professor Nazanin Derakhshan at the University of Reading says: 'In breast cancer there is evidence to show that anxiety and depression increases the risk of recurrence and mortality.

'More and more studies report this and we can't ignore it.'

Professor Flint says cortisol can help cancer cells evade the immune system, and to spread from the original tumour site to a secondary site.

'If you're stressed and this has lowered your immune system, or stress hormones are acting on the cancer cells themselves, then cancer cells can hide from the immune system and, depending on the type of cancer, spread to harder-to-treat areas like the brain, lungs, bone and liver.

'While the evidence on whether stress can cause cancer might be unclear, it's much clearer that it can cause an existing cancer to spread.'

There is likely to be more evidence on the horizon.

Oncologist Professor Charles Swanton, chief clinician for CRUK, said: 'Over the next five to 10 years, we may start to see an emergence of data testing the relationship between stress and cancer.'

But in the meantime, people who already have cancer, or those with a genetic mutation which increases their risk, should be given help to manage their stress levels, Professor Flint suggests.

This might involve talking therapies, medication, exercise or just 'looking after yourself' – all of which will be good for you, whether you have cancer or not.