The report found as a result of this lack of support, businesses are suffering.
It claims cancer is costing firms £1.1 billion each year in lost productivity and £1.2 billion in hard costs, including the costs of absence management, recruitment and training replacement staff.
The report found that businesses are losing talent and the resulting low morale has a negative impact on the bottom line.
It claims that despite businesses' initial efforts to support people returning to work, relationships can quickly, and unintentionally, breakdown due to a lack of regular and meaningful communication. Consequently the economy is missing out on £1.8 billion every year.
The report also claims that with greater support for employers and employees, by 2030, 136,000 people with cancer who want to work could, and they could contribute an additional £3.5 billion to the UK economy.
Joy Reymond, head of Unum's rehabilitation services, told HR magazine: "In society there is an ever increasing willingness to help sufferes and an understanding but the workplace just hasn't caught up.
"An employer has to be flexible. Those who aren't stand to risk losing their employee and a huge opportunity to help that worker could be lost."
Lesley Howells, Maggie's lead of research and centre head Dundee, said: "Many people living with cancer aren't able or don't want to work after treatment, but for those who can and choose to, it can be vital to their psychological wellbeing.
"People with cancer who use our centres tell us work can help build a sense of normality, structure, purpose and self-esteem. For these reasons, it's vital that businesses provide the right support for an employee returning to work with cancer."
Howells added: "This involves maintaining ongoing, meaningful communication and gaining and demonstrating better understanding. But businesses also need support - they are looking to the person with cancer to set the tone and need more guidance from them on what they need, as everyone's experiences are different."
Dr. Tyna Taskila, senior researcher at the Work Foundation, said: "Businesses need to wise up to the fact that return to work is now a realistic outcome for many people with cancer and make sure they have the right framework in place to help them.
"Many people still struggle to remain in the workplace after cancer and are at higher risk of early retirement, but businesses can play a key role in supporting them to stay in work."
The report estimated the number of people in employment with cancer by first estimating the number of people with cancer using forecasts of population and cancer prevalence rates in UK up to 2030. The number of people in employment with cancer was then estimated from this total using employment activity rates from the Labour Force Survey.
Below is a video from Maggie's and Unum about working after cancer