Council of Trade Unions president Helen Kelly has been diagnosed with lung cancer.
In an email to union delegates yesterday she said doctors had confirmed the cancer and she would now have more tests.
"Tonight the hospital rang and they have confirmed I do have a lung cancer," Ms Kelly said in her email.
"Nothing else is confirmed and I now have to have lots of additional tests and they have to test them etc etc.
"There is no confirmation of where else it might be or any prognosis."
She thanked delegates for their "beautiful support" and said she was looking after herself and feeling "remarkably well all things considered."
"I am writing this to you because rumours of my demise are exaggerated and I want to keep you all informed without the rumours."