The blast-proof panes in the Holyrood building cost �100m

The firm which installed the panes has been called back in after staff were told not to stick paper sunscreens to the glass.

A spokesman admitted contractors had said paper could cause a build-up of heat which could damage the glass.

He added that two panes had cracked since the building opened in October.

The parliamentary authorities believe there is something fundamentally wrong with the glass, installed by Drawn Metal of Leeds.

Anyone putting up paper blinds is being advised to keep them away from the surface of the glass Holyrood spokesman

The spokesman, who stressed the problem in the committee towers involves the inner layer of glass and not the bomb-proof pane, continued: "Parliament's experts believe the contractor who installed the glass is responsible.

"The company has claimed the heat build-up associated with temporary paper blinds has caused the cracks."

He continued: "We reject this idea entirely but for the time being anyone putting up paper blinds is being advised to keep them away from the surface of the glass."

The Holyrood windows have been plagued with problems.

They were difficult and expensive to manufacture. As many as 150 of the 500 windows have had to be replaced.