The shutdown, revealed in a report prepared for Wyre Council, would take place in the winter of 2017/18 and would see all services between Blackpool, Poulton, Kirkham and Preston replaced by buses.
Network Rail has promised to electrify the route by May 2018 and is also replacing outdated signalling and other infrastructure along the line.
The work is expected to include a significant rebuild at Blackpool North – including the lengthening and straightening of platforms – a remodelling of the railway at Kirkham and Wesham and the resignalling of the stretch between Poulton and Blackpool which is still operated using semaphore signals and bells.
Network Rail today said it could not confirm any dates or timescales for the closure but made clear disruption in some form was inevitable.
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The projected dates were revealed in a report, due to be presented to members of Wyre Council tomorrow.
It reveals Network Rail has been in contact with the authority with regard to the closure, without giving any specific dates.
In the report, leader Coun Peter Gibson said: “Significant engineering works will be required to facilitate electrification and there will inevitably be some disruption to train services and to residents who live in close proximity to the route.
“The works will result in an 18-week full line closure over the winter of 2017-18, and significant disruption to evening and weekend services from September 2016 to May 2018.
“I acknowledge that much of this disruption is inevitable and the electrification of the line is, in itself, a good and welcome improvement.
“Officers at Blackpool, Fylde and Wyre are, however, working with Network Rail to minimise disruption to passengers and residents over this period.”
Network Rail was unable to confirm specific dates or the planned length of the closure, insisting talks were still ongoing with rail operators and local authorities.
But the firm made clear there would have to be a lengthy closure.
A spokesman said: “We have always said that to deliver the significant benefits of an electrified railway between Blackpool and Preston there will have to a lengthy period of time – during the off-season months – when the railway has to close to carry out the upgrade.
“The closure will be carefully planned to cause the least amount of disruption to passengers and Blackpool and we are working with train operators and the local authority to finalise the dates.”
Business leaders and leading figures in the tourism industry have welcomed the upgrade works but admit to concerns over the potential for over-runs.
Hugh Evans, deputy chief executive of the North and West Lancashire Chamber of Commerce, said: “We’ve been pressing the case for full electrification of this line for some time and welcome the news that work will finally be completed by 2018.
“However infrastructure projects of this type and complexity have a habit of over-running and so Network Rail must do everything possible to ensure that the 18-week closure period is strictly adhered to even if mean keeping the work going 24/7.”
Hotelier Claire Smith, president of Stay Blackpool, said: “Do we want to be cut off from Preston? No.
“The good thing is that if this is right it’s not during the season.
“Four and a half months is a long time and we have to ensure that during any closure buses used in place of services are fit for purpose.
“All that being said, Blackpool will benefit long term from the improved services and hopefully better links to the big cities.”
Blackpool South MP Gordon Marsden is hoping the timescale revealed by Wyre Council is a sign Network Rail is close to finalising plans for the long-awaited upgrade.
He said: “It would be some relief – after considerable delay – if Network Rail is finally in a position to sort out a timetable for this very necessary work.
“There are supplementary questions that I would hope to ask.
“They include the implications for replacement transport, particularly for those with disabilities.
“It would not be satisfactory or acceptable for people with disabilities to be placed on transport that was not compliant with legislation.
“I know there are outstanding issues with regard to what work will be carried out at Blackpool North station. I know Blackpool Council has made robust representations to Network Rail on the issue.
“It would be crazy and a dereliction of duty if Network Rail were to pass up the opportunity to carry out work at the station.”
The shutdown is also expected to impact South Fylde services between Kirkham, Lytham, St Annes and Blackpool South, although Network Rail refused to confirm details.
Coun Tony Ford, chairman of the South Fylde Line Community Rail Partnership is hopeful the inconvenience will be short-lived.
He said: “Closure of the line for such a long period will obviously cause some inconvenience to passengers using the Blackpool North to Preston route.
“However, passengers will, I am sure, be given plenty of notice and more detail about the closure nearer the time so enabling them to make alternative travel arrangements.
“This closure is crucial to enable investment and improvements to this particular route.
“I welcome the investment in railways and the improvements they will bring to stations and journey times and provide passengers with a better travelling experience on newer rolling stock brought about by electrification.
“Passengers want better facilities and better trains so inevitably they have to shoulder some of the problems associated with large-scale engineering works of this nature. I am sure a rail replacement service will be provided.”