A legal action has been launched over the debacle of Glasgow's "Squinty Bridge", amid allegations that sub-contractors used faulty steel which did not meet recognised British standards.

A year after the collapse of a support cable forced the closure of the landmark bridge for six months, legal papers show the South Yorkshire-based steel supplier Macalloy is also accused of poor manufacturing and allegedly providing metal which failed to match test certificates it had supplied.

Watson Steel, fabricator of the £20.3m bridge, officially known as the Clyde Arc, is suing Macalloy for £1.8m.

Watson was forced to replace all 14 of the 35-metre supporting hangers on the bridge after one crashed on to the carriageway on January 14 last year, with immediate investigations finding a second was also cracked.

The bridge, funded by Glasgow City Council with Edmund Nuttall as the lead contractors, was just 15 months old and had won one of Scotland's leading awards when it was deemed unsafe.

According to the court papers, Watson claims that the metal used in the sections which connected the four-tonne hangers to the frame did not match any grades of steel recognised in the UK for general engineering purposes and alleges that it was significantly more brittle than had been specified by Macalloy.

Certificates relating to the steel supplied by Macalloy, which was responsible for the testing of components before delivery to ensure that they met the specification, allegedly failed to match results of tests on the steel in the bridge forks taken after the incident, Watson claims.

Watson Steel alleges that traces of other metals were found in the hangers' forks but that these had not been mentioned in the test certificates, while poor manufacturing of the fork is also cited as having contributed to the failure. Watson claims this contributed to the weight and force of the hangers being distributed unevenly, with the result that stress built up.

It is understood Macalloy has yet to respond to Watson's claims and the company declined an invitation to comment on the allegations.