Mr Chavez had not appeared in public since he returned to Venezuela last month after cancer treatment in Cuba.
An emotional Nicolas Maduro made the announcement on Tuesday evening, flanked by leading Venezuelan political and military leaders.
Earlier, he said the 58-year-old Venezuelan leader had a new, severe respiratory infection and had entered "his most difficult hours".
One of the most visible, vocal and controversial leaders in Latin America, the former army paratrooper won the presidency in 1998 and had most recently won another six-year presidential term in October 2012.
Last May, he said he had recovered from an unspecified cancer, after undergoing surgery and chemotherapy in 2011 and a further operation in February 2012.
However, in December 2012, he announced he needed further cancer surgery in Cuba, and named his Vice-President, Nicolas Maduro, as his preferred successor should the need arise.
Mr Chavez remained out of public view, finally returning to Venezuela in February.
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Edited by Scottish⑦Canuck, 05 March 2013 - 03:19 PM.