Britain's former prime minister Margaret Thatcher was released from hospital after staying overnight for checks after feeling unwell on Friday.

"She's had the tests but we won't be releasing any more information until we've spoken to her private secretary," the hospital spokeswoman told Reuters

Thatcher has appeared only rarely in public in recent years after suffering a series of small strokes in 2002 which forced her to cancel her busy public speaking schedule.

As Britain's first and so far only female prime minister, Thatcher was dubbed the "Iron Lady", earning a reputation as one of the most formidable politicians of her era. She came to power in 1979 and governed for more than 10 years before being ousted by members of her own party. She retired from the House of Commons (lower house) in 1992.