It said the one degree northwards extension to the zone, which was set up to protect marsh Arabs at the end of the Gulf War, would take effect at noon Baghdad time today.
The safe haven in northern Iraq, which aims to protect Kurds and which was also set up at the end of the war in 1991, remains unaffected, foreign office officials said.
They said the northern limit of the southern no-fly zone, which like its counterpart in the north is patrolled by British, French and US aircraft, would move from the 32nd to the 33rd parallel.
No extra aircraft are expected to be required.
The no-fly zone in southern Iraq will be extended to allow the allies operating there to have a greater opportunity to control more of Saddam's airspace and bring greater control over him, British defence secretary Micha-el Portillo told BBC radio.
Britain said Baghdad had been informed of the intention to extend the zone.British officials indicated that the extension was intended to be permanent.
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United Nations approval was not expected to be sought since both safe havens were set up by the international community in support of UN security council resolution 688 to monitor the activites of the Iraqi military forces.
British Prime Minister John Major's office said the decision had been taken in response to the recent massive buildup of Iraqi military forces and the attack on the Kurdish city of Arbil, which lies within the northern no-fly zone.
Asked why the allies had decided to enlarge the southern zone rather than the northern area, a British official said only; It was a military decision.
The statement from British Prime Minister John Major's office said the extension of the no-fly zone was intended to strengthen our ability to keep (Iraqi military) forces under close surveillance and to make clear to Saddam Hussein that such persecution of the Iraqi civilian population and such reckless behaviour are intolerable.