Television footage and CCTV recordings clearly show the culprit hurling the missile - as shown in this BBC picture - which landed in the penalty area.
United have vowed to do all in their power to ensure that the supporter, when identified, is banned from watching the Reds.
They have also urged other fans to co-operate with Hampshire Constabulary in identifying the culprit.
A statement from the club says: "Manchester United condemns the actions of the the spectator who threw the flare onto the pitch. The club strongly hopes that TV evidence will enable him to be identified.
"Once that has happened, United will take swift and appropriate action against him to make sure he will not be able to stain the club's reputation in such a way again."
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big dick, sale manchester (14/03/2005 at 10:20)
harsh talk of life bans lets keep a sense of perspective
yep haul the lad in give him a warning, one things for sure he`ll never do it again.
it should be left at that