Avanti West Coast has been ordered to stop releasing tickets just a few days ahead of travel or face "formal measures".
The train operator has been warned by rail regulator the Office of Rail and Road (ORR) to submit an improved recovery plan for producing timetables by February 2.
Avanti West Coast runs trains on the West Coast Main Line between London Euston and Glasgow Central with branches to Birmingham, North Wales, Liverpool, Manchester and Edinburgh.
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Passengers wanting to book tickets for weekend travel this month have only been able to purchase tickets a few days in advance, leading to claims that many people are being denied cheaper tickets.
Weekday tickets have also been released far later than the 12-week booking window normally used by operators.
Avanti West Coast initially made "reasonable progress" against an improvement plan created in September 2022 with passengers given more notice to book travel in the run-up to Christmas but the "position has deteriorated" this year, according to the ORR.
Current plans for February show a "better picture for weekday travel, but still fall short of passengers' needs for weekend journeys", the regulator stated.
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Stephanie Tobyn, director of strategy, policy and reform at the ORR, said: "Passengers are rightly frustrated that this situation has deteriorated so quickly after a steady start to the recovery plan late last year.
"By 2 February we expect Avanti to identify what has gone wrong and set out how it intends to get back to releasing timetables in normal industry timescales so that passengers can plan and book journeys with greater confidence.
"Failure to produce an acceptable plan or to deliver improvements may lead to more formal measures."
Avanti West Coast – a joint venture between FirstGroup (70%) and Italian state operator Trenitalia (30%) – was given until April 1 by the Department for Transport (DfT) to improve its services when it was awarded a short-term contract extension in October 2022.
It emerged on Monday that the operator cancelled the equivalent of about one in five services during the four weeks to January 7.
The company said "performance has steadily improved" since then.
Earlier this month, BusinessLive reported that Avanti West Coast had handed £13.5m to its shareholders during its latest financial year.
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The Companies House documents also showed that Avanti's turnover increased from £894.8m to £978.2m in the year while its re-tax profits dipped from £16.6m to £12.3m.
An Avanti West Coast spokesman said: "Unfortunately, some tickets for Saturdays and Sundays have only been available at short notice due to having to write unprecedented numbers of bespoke timetables to accommodate industrial action and engineering work which has stretched industry train planning resources.
"We know this causes huge uncertainty and inconvenience for passengers, and we’re sorry for that.
"We’re working with our colleagues at Network Rail to get tickets on sale as soon as possible and are encouraging passengers to sign up for ticket alerts.
"We’re also working closely with the ORR and keeping them informed.
"Weekend tickets are now on sale up to mid-February, and by mid-March we’ll be selling weekend tickets six weeks out."
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