A British jury has been told that a teenager accused of plotting a Columbine-inspired school massacre tried to buy an ingredient for a homemade bomb.

A witness told jurors at Manchester Crown Court on Thursday that 18-year-old Matthew Swift had asked her to buy him hydrogen peroxide, a common household chemical.

Classmate Phillipa Knowles says Swift said he wanted it to make explosives.

Prosecutors say Swift and 16-year-old Ross McKnight planned to bomb a shopping center before killing teachers and students at their school on the 10th anniversary of the April 1999 massacre in Colorado.

They say the pair dubbed the plot "Project Rainbow" and boasted it would be "the greatest massacre ever."

The defense denies the charges but has not yet argued its case.