The charities were unaware of the timing of each other's campaigns, both of which focus on skin cancer.
With Macmillan Cancer Relief planning its skin cancer awareness drive next month, such an unco-ordinated approach has raised fears that the message is being diluted.
Macmillan spokeswoman Kirsty Warwick said: "Some journalists are already telling us there are too many awareness campaigns."
Cancer Research's increasingly close relationship with the Government is likely to intensify the situation. "The Department of Health wants people to do its awareness for them but with money from them," said Sara Hiom, Cancer Research UK's science information manager (health).