World Bulletin / News Desk
Sadr said in a Sunday statement that the British troops would be considered as "invaders" if they took part in any operations in Iraq.
Last week, British defense minister Michael Fallon had said the U.K. would increase the number of the soldiers to be sent to Iraq to combat ISIL to 500.
In early July, Sadr said American soldiers would "be a target for us" after U.S. Secretary of Defense Ashton Carter declared that the Pentagon would send 560 more troops to Iraq.
Sadr, who has thousands of supporters, fought U.S. troops several times during the 2003 occupation of Iraq.
Iraqi forces are preparing to launch an offensive to retake Mosul from ISIL, which overran the northern city in early 2014.
Iraq has suffered a devastating security vacuum since mid-2014, when ISIL captured Mosul and overran large swathes of territory in the northern and western parts of the country.