Downing Street and opposition leader David Cameron both criticised plans by Islam4UK to march through Wootton Bassett, Wiltshire, as "inappropriate" and "unacceptable".
Civic leaders also spoke out against proposals by the group - a branch of the extremist al- Muhajiroun movement - to parade along the high street with empty coffins symbolising the conflict's Muslim victims.
The small market town has unwittingly become a public stage for soldiers and relatives to pool their grief for the fallen soldiers who pass through in coffins after being repatriated at nearby airbase RAF Lyneham.
Downing Street today said any protest march which offended the families of soldiers killed and injured in Afghanistan would be considered "completely inappropriate" by the Prime Minister.
Gordon Brown's spokesman issued the warning after group leader Anjem Choudary vowed to continue with the plans.
Mr Brown's spokesman said: "We do not yet know what his plans are. The only thing he seems to have said is that it is an idea he is considering. He would need to have the appropriate contact with the right people in Wootton Bassett to secure approval.
"The Prime Minister's view would be obviously that anything that is considered to be offensive to, or of concern to, families of troops wounded or killed in Afghanistan would be completely inappropriate."
David Cameron condemned the march and added that Mr Choudary came close to encouraging "hatred, extremism and violence".
Speaking during a visit to the Rowcroft Medical Centre in Stroud, Gloucestershire, Mr Cameron said: "I think that it is completely unacceptable for that (the march) to happen.
"I think this group are just saying that because they want to get some cheap publicity.
"I think their views are completely reprehensible to the overwhelming majority of not just the British public, but British Muslims as well.
"Anjem Choudary is one of those people who needs to be looked at seriously in terms of the legality of what he's saying because he strays, I think, extremely close to the line of encouraging hatred, extremism and violence."
North Wiltshire MP James Gray said the thousands who appear regularly would say the protesters were "foolish people making a silly point".
Civic leaders have begged Mr Choudary to reconsider his proposal and a Facebook site dedicated to preventing the march has already attracted more than 120,000 members.
The town council said it had yet to be consulted on the proposals.
Mayor of Wootton Bassett, Steve Bucknell, said: "The people who attend the repatriations no doubt have a wide range of views about the conflicts, but those views are not voiced in our High Street, out of respect to those who have lost their lives and those who grieve for them."
Mr Choudary, who wants a withdrawal of troops from the country, re-stated his reasons for the march on his website.
In a long letter entitled "To the families of British soldiers who have fallen", he also mis-spells the name of the market town.
Mr Choudary said: "It is worth reminding those who are still not blinded by the media propaganda that Afghanistan is not a British Town near Wootton Basset (sic) but rather Muslim land which no one has the right to occupy... The procession in Wootton Basset is therefore an attempt to engage the British publics minds on the real reasons why their soldiers are returning home in body bags and the real cost of the war."
The walk will not coincide with the return of a dead soldier's body, Mr Choudary added.
Wiltshire Police warned in "exceptional circumstances" the authority could apply for an order prohibiting such a march.
A statement from the constabulary said: "If the march or procession is believed to be likely to result in serious disorder, disruption or damage, then the police can impose conditions upon the organiser.
"In exceptional circumstances, the police may apply to the local authority for an order prohibiting such a march."
Elsewhere today, a court heard a group of Muslim men told British soldiers at a homecoming parade to "burn in hell" and branded them rapists, murderers and baby killers.
Angry scenes broke out during the parade in Luton for the 2nd Battalion The Royal Anglian Regiment, known as The Poachers, on March 10, 2009.
Seven men, all from Luton, appeared at Luton Magistrates' Court charged with using threatening, abusive or insulting words or behaviour likely to cause harassment, alarm or distress. They deny the charges.
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