Demonstrators break into Cabinet Office

The Government suffered another major security embarrassment when four anti-war activists broke into the Cabinet Office.

Three women and a man were arrested after walking into the key building at Whitehall and demanding to speak to "those responsible for the killing in Iraq".

The three women were inside for 20 minutes before the alarm was raised at lunchtime yesterday, it is understood.

The breach is particularly embarrassing as it came on the day of the Queen's Speech in Parliament when Westminster was packed with armed police.

The protesters strolled past security at the main entrance before donning outfits covered in dried blood to launch their " diein" protest.

They had staged similar protests outside Downing Street and Parliament earlier in the day.

They were arrested on suspicion of burglary and were being questioned at Charing Cross police station last night. Their protest comes after a string of high-profile incidents at Whitehall.

Two months ago, five people sparked a security review when they broke into the Commons to demonstrate against the ban on foxhunting. It was the first time Parliament had been invaded for 350 years.

In May, two members of Fathers4Justice threw flour bombs at Tony Blair during Prime Minister's Questions after obtaining passes for the Commons.

And two Greenpeace protesters scaled Big Ben in March.