Catherine Zeta-Jones admits it is tough watching her husband battle throat cancer.

The 40-year-old actress is saddened by the toll gruelling radiotherapy and chemotherapy is having on Michael Douglas, who announced last month he was undergoing treatment after discovering a tumour at the base of his tongue.

She said: "The hardest part is seeing his fatigue, because Michael is never tired."

The couple have two children together, Dylan, 10, and seven-year-old Carys and the 'Wall Street: Money Never Sleeps' actor - whose son from a previous marriage, 31-year-old Cameron, is currently serving a five-year jail term for drugs offences - insisted on breaking the news of his condition to them himself.

Catherine added to People magazine: "Daddy sat them down and told them he has cancer now."

Despite admitting his cancer is at an advanced stage, 65-year-old Michael - who was diagnosed with the illness after complaining of persistent ear and throat pain - is confident he can beat the disease, though he is finding treatment hard.

He said: "It's a fight. It really knocks you out."

Catherine added: "If there's anything Michael has, it's strength."