A high-speed train was allowed to cross a storm-damaged viaduct minutes before engineers discovered a large crack in the structure.

The Lamington Viaduct on the West Coast Main Line over the River Clyde was severely damaged by flooding and heavy rain during Storm Frank on Hogmanay.

Investigators have found engineers inspected the southbound track and removed a speed restriction before spotting a fault on the northbound side as a high-speed train passed through.

Network Rail said it is working closely with the Rail Accident Investigation Branch (RAIB) as it carries out its inquiry into the incident.

Several trains passed over Lamington Viaduct, South Lanarkshire, on the morning of December 31 after the ground beneath a viaduct pier had been eroded by river scour.

The viaduct carries the railway, running between Carstairs and Lockerbie, across the River Clyde. It is a section of the West Coast Main Line between Glasgow and London where passenger trains are permitted to travel at up to 120mph.

Although there were no injuries or damage to trains, the viaduct was seriously damaged and the main line is expected to remain closed until March 2016 while it is repaired.

At 7.35am, the driver of the southbound Virgin West Coast service from Edinburgh Waverley to London Euston reported a dip in the track.

In response, signallers restricted the speed of trains until track maintenance staff arrived on site.

After inspecting the track and watching a southbound train pass at low speed, the maintenance staff removed the speed restriction.

They remained on site to undertake minor remedial work. After observing unusual track movement at 8.40am as the Virgin West Coast service from Crewe to Glasgow Central passed at high speed, they immediately reimposed temporary speed restrictions on trains in both directions.

They blocked the line to all trains a few minutes later at 8.53am after observing a large crack in pier number two, which is one of three viaduct piers located in the river.

Subsequent investigations have revealed a large hole beneath pier number two, subsidence of the pier and damage to three of the steel bearings that support the bridge deck.

The damage was discovered following exceptionally high water levels and fast flows in the River Clyde due to Storm Frank. On January 10, after the viaduct had been closed, a disused part of pier number 3 collapsed.

A statement from the RAIB said: "Our investigation is independent of any investigation by the railway industry or by the industry’s regulator, the Office of Rail and Road.

"We will publish our findings, including any recommendations to improve safety, at the conclusion of our investigation. This report will be available on our website."