The charity says the coronavirus pandemic has had a devastating impact on the mental and physical health of people living with cancer.
Nine per cent of UK cancer patients have experienced panic or anxiety attacks, or even suicidal thoughts because of the virus.
And around 90,000 people with cancer say they won't feel safe enough to leave the house until a vaccine or effective treatment is widely available.
One in seven have experienced a decline in their physical health during lockdown.
"For many people, it is more frightening to be diagnosed with cancer now than during any other time in recent history," said Lynda Thomas, CEO at Macmillan Cancer Support.
"On top of the impact of a cancer diagnosis, patients now feel lost in lockdown. They are having to contend with the two potentially deadly Cs all at once, including uncertainty around treatment, shielding restrictions and isolation from loved ones, as well as concerns about their increased risk of contracting the virus."
Michelle Rowe, 21, was diagnosed with non-Hodgkin lymphoma for the second time in April.
"I would get really bad pains in the night and have to go to A&E, and I literally would have a breakdown. I wasn't able to go in with my family as well and I didn't trust other people to keep a distance from me," she told Sky News.
"Obviously, being extremely, clinically high-risk, you cannot be in contact with anyone and I feel like people aren't always conscious of who's around them."
Ms Rowe, who has been shielding since April, says she does not feel confident about venturing out of her home despite government assurances that the chance of encountering coronavirus in the community continues to decline.
"I can't imagine my life going back to normal for a very long time. It's part of the reason why it's so difficult to cope."
"I'll go down my Instagram feed and people are almost bragging about breaking social distancing measures and putting it as a caption. I know if I got coronavirus I could potentially die and it's something I have to think about every day."
Fifty thousand people were able to continue cancer treatment in March and April as hospitals were hit with the first wave of the virus.
As a new specialist hospital opened in Liverpool on Monday, NHS Chief Executive Sir Simon Stevens said: "NHS staff have done everything in their power to make sure that the NHS can continue to care for patients who needed treatment and this has happened, which is testament to their incredible hard work and dedication."
On 22 June, the government set out a series of steps for further relaxing shielding guidance.
From 6 July, those who have been shielding will be able to meet in a group of up to six people outdoors, while maintaining strict social distancing. Other rule changes say they will not need to observe social distancing within their own households.
A Department of Health and Social Care spokesperson told Sky News: "Even as we fight this unprecedented pandemic, the NHS has continued to treat cancer patients as a priority with vital tests and treatments going ahead in safe way for thousands of patients."