Commuters travelling from Belfast to Dublin have been warned that they face travel chaos from today following the collapse of the railway line just north of the Irish capital.

Around 20 metres of the viaduct, which spans the Broadmeadow estuary on the outskirts of north Dublin, collapsed into the water at around 6.30pm on Friday.

A major disaster was averted as no trains were travelling over the viaduct at the moment of the collapse. But it has emerged that a 4.10pm service from Belfast Central station would have been scheduled to pass over the line at around 6pm, shortly before the collapse, while a 7pm service from Dublin to Belfast had been due to cross the viaduct a short time afterwards.

The collapse was reported by a quick-thinking train driver who raised the alarm after spotting subsidence on the track as he crossed over on Friday evening.

Irish rail company Iarnrod Eireann said it expected the line, which runs between the towns of Malahide and Donabate, to be closed for up to three months while inspections and repair work are carried out, although some reports have speculated it could be as long as 11 months following a similar accident in Cahir in 2003.

The closure will lead to major travel disruption for the thousands of passengers who use the line every day.

Northern commuter services are among the worst affected, with passengers using the Belfast to Dublin Enterprise service only able to travel as far as Drogheda before being transferred to buses for the remainder of their journey to Connolly Station in Dublin.

A spokeswoman for Translink apologised for the inconvenience and urged passengers to allow extra time for their journeys.

“Passengers can keep updated on our call centre number on 028 9066 6630,” she said.

Around 10,000 passengers daily will be affected by the disruption, with people travelling between Dublin city and all stations north of Malahide being advised to take the bus.

Conall McDevitt, managing director of Belfast PR company Weber Shandwick, said he would be badly affected by the disruption.

“My job takes me down to Dublin every week and I would use the train as often as possible to get there,” he said.

“I’m due there this week and it is going to be a nightmare. All the northern traffic in the city, of which there is a lot, is now off the train and on the road. It’s a toss-up between taking the car or the bus.

“It will be misery for people for the next three months while they fix it.”

Pressure group Rail Users Ireland said it was “extremely concerned” following the collapse.

“This viaduct carries some of the busiest commuter trains in Ireland, it is nothing short of a miracle that the collapse did not result in a serious accident and loss of life,” a statement from the group said.

“As the bridge was built, owned and maintained by Irish Rail, they must take complete responsibility for the collapse of the viaduct and the resultant disruption passengers will experience for several weeks if not months.”

Irish Rail spokesman Barry Kenny said it would cost several million euros and take at least three months to repair the viaduct but this would be made more difficult by strong tides.

“The scale of the potential disaster was enormous. The fact that nobody was hurt and there wasn't a derailment doesn't take away from the fact that this was very close to being a very serious tragedy,” he said.

The incident happened on the line which has seen the heaviest freight use of any railway section in the Republic, with trains weighing up to 800 tonnes taking iron ore from Tara Mines using the line in the past.

Around 90 trains a day pass over the bridge. When the bridge collapsed the railway control centre was also alerted via its computer system. The viaduct and bridge were built in the late 60s to replace a bridge built in 1848.

Mr Kenny said that different sections of railway tracks were walked three times a week; there was a major structural inspection every two years, and this particular viaduct had been inspected on Tuesday.

As well as an investigation by Iarnrod Eireann, the rail accident investigation unit of the Republic’s Department of Transport was also carrying out an inquiry.

Thousands of GAA fans who travelled from Tyrone to Croke Park for the All-Ireland football semi-final yesterday were affected by the rail closure.

Information for travellers is available by visiting www.irishrail.ie.