Speculation over the extent and magnitude of Sonia Gandhi’s health has been renewed following a surprising decision by the head of India’s Congress Party to deliver a speech.

Gandhi, the widow of former Prime Minister Rajiv Gandhi, had planned to make her first public appearance since undergoing surgery in New York in August for some undisclosed medical illness.

Party officials claimed she cancelled an appearance in the northern state of Uttarakhand because she had “viral fever,” however Congress has repeatedly refused to provide any details of her medical condition. She was out of the country for a whole month.

Some Indian media reports have speculated that she has some form of cancer.

Although the 64-year-old Italian-born Sonia holds no elected office, she is widely regarded as being the most powerful and influential political figure in India. Indeed, the current Prime Minister Manmohan Singh was nominated by her.

Sonia is widely believed to be grooming her son, MP Rahul Gandhi, 41, to become the next Prime Minister.

DH Pai Panandiker, president of RPG Foundation, a New Delhi-based research firm, told The Daily Telegraph newspaper of Britain: “It is an absolute certainty that [Rahul] will be the next leader of Congress, it is just a matter of when. He is from the right family and has been groomed for the prime minister’s job in the last few years.”

Sonia is the daughter-in-law of Indira Gandhi.