The Stop the War Coalition said that the 11 people had been contacted by police and told that only six of them could enter Downing Street and that a wreath could not be left outside. The official spokesman of the prime minister, Tony Blair, said: "All that we have done is point out to the families that there is nowhere formally to lay a wreath in Downing Street.

"If they wish to bring a wreath to Downing Street and give it in through the door, that's fine by us. It's entirely a matter for them. All we did was point out there was no memorial as such in Downing Street at which to lay a wreath."

Among the relatives is Rose Gentle, whose 19-year-old son. Gordon, a Royal Highland Fusilier, was killed by a roadside bomb near Basra in June. She was hoping to meet Mr Blair in person to press home the demand for troops to be withdrawn.

Other people joining the group included Reg Keys, whose son Lance Corporal Thomas Keys, a member of the Royal Military police, was murdered by a mob last year, and James Buchanan, from Arbroath, whose son Gary is serving in the Black Watch.

The protest comes on the day that bodies of the three Black Watch troops killed in a suicide bomb last week were flown back to the UK from Iraq. Private Paul Lowe, Sergeant Stuart Gray and Private Scott McArdle, all from Fife, were killed in the so-called "triangle of death" last Thursday. The 850-strong Black Watch battlegroup that they were part of recently controversially re-deployed south of Baghdad after a request from US commanders to free up US marines for the unfolding attack on the insurgent centre of Falluja.

A fourth Black Watch soldier, Private Pita Tukutukuwaqa, 27, from Fiji, died when his Warrior armoured vehicle was hit by a roadside bomb north of Camp Dogwood on Monday.

As the battle for Falluja raged today, Black Watch continued to patrol the sector south east of the city to try and stop insurgents escaping.

Speaking from her home in Glasgow before leaving for London, Ms Gentle said she would request a meeting with Mr Blair. She said: "We just want to bring all the troops back home. Hopefully, he will listen to us this time - especially when all the Black Watch boys are being killed."

In August, Ms Gentle stormed out of a Downing Street meeting with the deputy prime minister, John Prescott, and said he had talked "rubbish".

The families will be joined by Dante Zappala, a member of the US pressure group Military Families Speak Out. His brother, 30-year-old Sergeant Sherwood Baker, was killed in Baghdad while serving with the US army for the Iraq Survey Group.

Meanwhile today, Mr Blair insisted that no decision had been made on the future of Scottish regiments including the Black Watch, after claims that he was on the brink of a U-turn.

Mr Blair clashed angrily in the Commons with the Tory leader, Michael Howard, who said his treatment of the Black Watch and soldiers' families at a time when they were in action in Iraq was "shameful".

The six Scottish regiments are due to be trimmed to five and then merged under plans announced by the defence secretary, Geoff Hoon.