Rail passengers who like to plan and book train journeys well ahead face unwelcome surprises for the remainder of the year: they may pay more than they need, or discover that their scheduled service is cancelled.

Network Rail normally publishes its schedules for engineering work more than 12 weeks ahead, allowing train operators to finalise their timetables and start selling Advance tickets.

Passengers who book early normally get the lowest fares. Some train operators aim to sell tickets up to 24 weeks in advance.

But for six months from 20 May, the infrastructure provider is reducing the amount of notice it gives to train operators, meaning timetables will be published later than normal.

While Network Rail aims to give six weeks’ warning, some schedules, particularly for weekends, may only be known less than four weeks ahead.

Disruption is particularly prevalent in north-west England, where electrification work between Manchester and Preston is behind schedule. The project is over a year late, with delays blamed on uncharted mine workings.

New Thameslink train services on the East Coast main line from Peterborough and Cambridge via central London are also being slotted into one of the busiest routes in Europe.

Timetables change in May and December each year, usually with a modest number of amends. But the May 2018 timetable change is said to be “the largest in living memory”, with four million services rescheduled – seven times the usual number of alterations.

Jo Kaye, managing director of System Operator for Network Rail, said: “This decision hasn’t been taken lightly, but there is significant change required in the timetable resulting from unprecedented investment, combined with short‐term changes to projects and plans, meaning that our timetable planning resource is at more than full capacity.”

Train tickets for 20 May onwards would normally go on sale from Sunday 25 February, and many will be available as usual.

But the usual window for Advance tickets on some routes will be reduced, meaning travellers could pay more than they expect. Alternatively, they could find that a train they book in good faith is either not running or disrupted, with a rail replacement bus service operating.

The announcement is likely to benefit coach operators such as Megabus, National Express and Sn-Ap.

The Rail Delivery Group (RDG), representing Network Rail and the train operators, said: “While most timetables and tickets will still be available at the usual 12 weeks before travel, these will not be absolutely confirmed until closer to the date – usually six weeks beforehand – so may be subject to more short notice changes than normal.”

The RDG said only two per cent of passengers will be affected; the remainder use season tickets, walk-on fares or book less than six weeks before travel.

But it concedes “some passengers will be affected and the industry is working together to ensure that nobody loses out“.

In cases where the normal 12-week booking horizon cannot be met, train operators have agreed to offer the same range of discounted tickets – starting at the usual lowest fares.

If trains are cancelled, passengers who decide not to travel will be given a full refund. And travellers who find that they have overpaid, because cheaper Advance tickets later go on sale, are being told to re-book at the lower fare and then request a fee-free refund on their original ticket.

The regulator, the Office for Rail and Road (ORR), said it was “concerned about the impact” on passengers and freight customer.

The ORR’s chief executive, Joanna Whittington, said: “We have already called in Network Rail to explain the arrangements it has put in place to minimise the impact on passengers and freight customers. We will monitor its progress against the proposed recovery plan.

“We will want to ensure that any lessons learned from this are properly addressed for the future.”

Anthony Smith, chief executive of the independent transport watchdog, Transport Focus, said: “Passengers have been badly let down and will be deeply frustrated that they can’t make plans with certainty. They are also at risk of paying more.”