More than 100 peace protesters broke into a major British defence site this morning, according to pressure group Greenpeace.

Greenpeace said its activists entered Menwith Hill spy base, near Harrogate, North Yorkshire, at 5am to protest against United States plans to use it as part of its Son of Star Wars national missile defence programme.

North Yorkshire Police confirmed they were attending an incident at the base but said they could give no further details.

Greenpeace said three groups of protesters were occupying three areas within the high security site.

It claimed one group of 50 activists, some carrying flags with the message ''Star Wars Starts Wars'' and others dressed as missiles, walked straight through the main gate to the complex playing the theme tune to Mission Impossible.

The other two teams scaled three-metre-high razor wire fences to enter the base.

Greenpeace UK executive director Stephen Tindale, who is at Menwith Hill, described President George Bush's proposed missile defence programme as ''a disaster'' and called on Prime Minister Tony Blair to refuse Mr Bush permission to use the UK-based sites at Menwith Hill and Fylingdales, North Yorks.

He said: ''Bush can't install the system without Tony Blair's approval.

''We urge Mr Blair not to kowtow to Bush on such a crucial issue. He must say no to UK involvement.''

Protester Helen Wallace, speaking on BBC Radio 4's Today programme said a number of people had been arrested at the site.

She said:''I am chained to a water tower in the site. There are around 20 people here on top of the tower, and other people chained down below. ''Other volunteers from Greenpeace are elsewhere on the site highlighting Menwith Hill's potential role in Star Wars. ''We came in through the main entrance. There were a couple of police officers trying to stop us. ''A number of people have been arrested, however we are going to stay here continuing to make our point that Menwith Hill is part of Bush's dangerous Star Wars plan.''

Updated: 10.27 Tuesday, July 3