The beam came loose while MSPs were in the chamber

Structural engineers Arup are unable to say how long it had been missing - because they cannot find the bolt.

Their latest report also confirmed that another bolt holding the roof strut, which is being transported to St Albans in England for tests, had fractured.

MSPs have been unable to use the chamber since the beam came loose.

The Scottish Parliamentary Corporate Body (SPCB) is currently looking at the third update report from Arup.

'Undue stress'

In it, the structural engineers said: "We have been unable to find evidence of the whereabouts of the missing bolt, and cannot therefore determine for how long the bolt has been missing."

Arup said it was exploring various scenarios, including the suggestion that the lack of a bolt placed undue stress upon the remaining bolt, causing it to fracture and allowing the strut to become dislodged.

Arup will consider whether the bolt could have been damaged during its installation.

Engineers will also look into options for replacing the bolts.

MSPs discussed chamber business in the Hub

An earlier report from Arup recommended that the beam be taken to an accredited testing house to enable the failed bolt and its socket to be removed for metallurgical testing.

It left the Scottish Parliament at 1300 GMT on Saturday, bound for Stanger Materials Science in St Albans, Herts.

One of the Holyrood chamber's 60 specially laminated and steel-reinforced oak beams swung free on 2 March.

The strut almost crashed into a glass screen and ended up hanging 20ft above the Tory benches.

MSPs used the nearby Hub to debate chamber business last week, at an estimated cost of �40,000.