BAGHDAD, April 29 (Reuters) - A British soldier was shot dead while on patrol in the southern Iraqi city of Basra on Sunday, in what has been the bloodiest month for British forces in Iraq since the U.S.-led invasion in 2003.

Twelve have been killed in April, the highest number of casualties in a single month since March 2003, when 27 were killed in the opening days of the war.

A British military spokesman in Basra, Major David Gell, said the soldier was shot after he had dismounted from his vehicle during a routine patrol in the Al-Ashar district of Basra.

"This has been a particularly hard month for us," he said.

In London, the defence ministry said the soldier was from the 2nd Battalion The Rifles, but did not name him.

Analysts say that with British troops poised to begin reducing their 7,000-strong force in Basra to about 5,500 by the beginning of June, Shi'ite militias are increasing their attacks so that they can claim victory when they eventually pull out.

British bases have been struck almost daily by mortar fire and patrols hit by roadside bombs and machine gun fire.

In the worst single attack this month, four soldiers were killed when their Warrior armoured vehicle was blown up.

A total of 146 British troops have died in the Iraq campaign since the 2003 invasion. (Additional reporting by John Sinnott in London)

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