The under-fire rail operator Northern has been accused of breaking promises after it asked to extend the closure of a line in the Lake District following weeks of misery for commuters.

The company announced on Monday that it was suspending the Lakes line between Oxenholme and Windermere for two weeks as part of a mass cancellation of services.

But on Thursday night, Northern said it wanted to continue the suspension for a further two weeks, prompting calls for the transport secretary, Chris Grayling, to intervene.

Grayling told the House of Commons on Monday he was “profoundly unhappy” about the line closure and “not prepared to accept more than the current two weeks” of suspension.

The former Liberal Democrat leader Tim Farron, one of the MPs representing the area, described Northern’s request as a “disgrace” and called for the transport secretary to reject it.

Cumbrian businesses have warned the closure of the line risks doing irreparable harm to the region’s tourism industry.

The line, which was earmarked for closure as part of the Beeching rail cuts in the 1960s, is a vital link to some of the most idyllic spots in the Lake District.

Speaking to the Guardian on Friday, Farron said: “This iconic line that escaped the Beeching axe now faces death by Northern’s incompetence and the government’s inaction.

“Chris Grayling promised on Monday that he would block any extension to the Lakes line cancellation. He must keep his word and tell Northern to do its job and run trains.”

The Department for Transport has yet to return a request for comment, but BBC Radio Cumbia reported the department had not objected to the longer suspension. Grayling is facing questions about whether he approved the extension, after earlier indicating he would not.

Transport for the North, a statutory body that advises the government on strategy and investment, said it “reluctantly accepted” Northern’s request to extend the line closure on the basis that the operator said reopening the line as planned would cause further delays in Cumbria and beyond.

John Cridland, the TfN chairman, urged Northern to examine ways in which the line could be restored “by the beginning of July at the latest”.

“It’s disappointing that we are in this position, as communities and businesses rely on the Lakes line service,” he said.

“Northern must now do everything they can to get a train service running again on that line as soon as possible. This cannot go on for a prolonged period.

“Northern has said that passengers would be likely to experience further cancellations and unscheduled delays across the Lakes and wider network if services are reinstated. As the voice of the north’s communities, we cannot responsibly insist that they take that operational decision. The passengers come first.”