At least 40 people were killed and 60 wounded when a suicide bomber detonated himself in a crowd of Shia pilgrims south of Baghdad today, raising fears of further bloodshed ahead of the climax on Wednesday of a major holy festival.

The bomber targeted a tent in the town of Iskandariyah where pilgrims had been taking a rest en route to the holy city of Kerbala, 70 miles south of the Iraqi capital, to mark Arbaeen. Women and children, struck by ball bearings, were among the dead.

One woman, who suffered multiple injuries, said: "When we reached the area people invited us into a tent to take some rest and have some food. When we entered, there was a huge ball of fire and we saw people lying on the ground."

About 42,000 pilgrims had already passed without incident through the rural area of palm groves and small villages, which is dubbed the “triangle of death”. It was once an insurgency stronghold before a surge of American and Iraqi troops last year produced a dramatic drop in attacks.

The Arbaeen festival, to commemorate the end of a 40-day mourning period following the anniversary of the martyrdom of Imam Hussein, one of two revered Shia figures buried there, draws millions of people to Kerbala every year.

It is a soft target for suspected Sunni Arab insurgents, however, because many of the pilgrims prefer to walk to the city, believing that the effort will bring them greater spiritual reward. In scenes of carnage last year, 149 pilgrims were killed.

Anxious to try to prevent another wave of attacks, tens of thousands of Iraqi soldiers and police have been deployed to the area over the past few days.

Despite such precautions, a second group of pilgrims was attacked today by gunfire and grenades as they passed through a predominately Sunni Arab neighbourhood in south Baghdad. Three were killed and 36 wounded, police said.

In addition, several potential attacks have already been averted, including the discovery of a parked car loaded with explosives near Kerbala.

More than 4 million people are already gathered in the city, with many more on their way. A steady stream of pilgrims, some carrying green, black or red banners, could be seen walking out of Baghdad yesterday.

Away from the festival fears, Nouri al-Maliki, the Iraqi Prime Minister, flew to London today for a second round of medical tests. Aides described the process as a routine check-up and said that the Prime Minister was in good health.

On a previous visit over the New Year, Mr Maliki underwent a cardiac catheterization, a routine procedure in which a thin tube is inserted into an artery or a vein to check for heart problems.

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