The one-time Marxist who toppled the bloody dictatorship of Mengistu Haile Mariam in 1991 was in a life-threatening condition, a diplomat who asked not to be named said.
"He is in a critical state, his life is in danger," said the diplomat.
In Addis Ababa, however, government spokesman Bereket Simon denied reports that the 57-year-old premier who has held power for over two decades was ill.
"He is not in a critical state. He is in good condition," the spokesman told AFP.
In Brussels, the Ethiopian embassy refused comment. It had said earlier this week that reports Meles was being treated at a hospital were "false and wrong", and were a rumour created by "an interest group which has preoccupied itself in disseminating such untrue stories".
But several diplomats in Brussels said he had been undergoing regular treatment on a private basis at one of the city's major hospitals and had been in hospital for some days.
No information was available on his illness.
Questions surfaced about Meles's health when he missed a two-day African Union summit Sunday and Monday in Addis Ababa, apparently for the first time.
Meles's wife, herself a lawmaker, had declined to talk to reporters about her husband.
One of the last times Meles was seen in public was at the G20 meeting in Mexico on June 19.
Dozens of African heads of state visited Ethiopia for the summit, including newly elected Egyptian President Mohamed Morsi, the first to do so since an assassination attempt in Ethiopia on former president Hosni Mubarak in 1995.
Benin's president and current AU chairman Thomas Boni Yayi had said at the opening of the summit Saturday that the "unusual absence... cannot go unnoticed, because we know that Mr Meles is full of dynamism and leadership in our meetings".
Meles has been vilified by some as a dictator and praised by others as a visionary.
Last week, UN human rights chief Navi Pillay condemned a "climate of intimidation" in Ethiopia after a court jailed a journalist for 18 years for "terrorism" and 23 other reporters and activists for between eight years and life.
Pillay said the overly broad definitions in a July 2009 anti-terrorism law had resulted in "criminalising the exercise of fundamental human rights".
Born on May 8, 1955, Meles abandoned his medical studies before turning 20 to join the Tigray People's Liberation Front and fight Mengistu's regime.
In 1989, he took over the group's leadership and then forged a broader coalition with other regional movements to make up the Ethiopian People's Revolutionary Democratic Front (EPRDF), now the country's ruling party.
With US backing, the rebellion toppled Mengistu in 1991, a year after Meles abandoned Marxism.