Ministers say all options are on the table, but admit there is no indication when services to and from the capital may return to normal.
The operator has run fewer than half of its normal services since August 14 and blamed the reduction on “unofficial strike action” by drivers.
The firm says it normally runs around 400 trains per week with drivers voluntarily working on their rest days for extra pay but that “dropped suddenly to fewer than 50”.
Transport minister Trudy Harrison faced shouts of “when?” from Labour MPs after she made the comments in the Commons to a question about Avanti West Coast’s services that connect the West Midlands to London and Manchester.
Ms Harrison told MPs: “Avanti has reduced its timetable in response to the withdrawal of the rest day working. Reducing the timetable provided better certainty and reliability for passengers as it reduced the number of short-notice cancellations.
"The department continues to work closely with Avanti to monitor performance whilst it continues to review the demand data and the position regarding train crew availability to inform options to reliably increase services.
"An increase in services between Manchester and London remains an absolute priority and Avanti will continue to look for opportunities to support passengers and businesses along this route.”
Ms Harrison earlier said the West Coast partnership franchise agreement is due to expire on October 16, telling the Commons “all options” were on the table, adding: “Withdrawing Avanti’s contract is one of those options but we are bearing in mind all of the implications of that.”
Staffordshire Tory MP Sir William Cash said the strikes are “completely unwarranted” but “Avanti has got to get its act together soon”, saying he has been using the line for 37 years “and I’ve never seen it in such a state”.
Labour MPs from the Manchester area criticised services. Stockport MP Navendu Mishra said: “Chaos continues to blight the lives of thousands.”
Ms Harrison said the situation is “untenable, needs to be improved”.