To reach the conclusion, the US research team tested the impact of a form of vitamin C on the effectiveness of a range of anti-cancer drugs in tests on cancer cells in the lab.
Te analysis revealed that every drug they tested did not work as well if cells were pretreated with vitamin C as they did on untreated cancer cells.
Between 30percent and 70percent less cancer cells treated with vitamin C were killed, depending on the drug tested.
Follow-up tests on mice showed that while chemotherapy kept untreated cancer in check, tumours grew more rapidly in mice that were given cancer pretreated with vitamin C.
Some classes of cancer drugs produce molecules known as oxygen free radicals which can react with other molecules in the cancer cell, forcing its death.
In theory, vitamin C could mop up the free radicals, keeping the cancer cell alive despite chemotherapy treatment.
However, the researchers found the key was not that the nutrient was neutralising free radicals.
Instead, vitamin C appeared to protect tiny structures inside the cancer cells called mitochondria from damage.
Mitochondria effectively form the energy-creating boiler room of a cell, and if damaged can lead to its death.
"Vitamin C appears to protect the mitochondria from extensive damage, thus saving the cell. And whether directly or not, all anti-cancer drugs work to disrupt the mitochondria to push cell death," BBC quoted lead researcher Dr Mark Heaney, as saying.