National Express has debts of nearly £1bn
National Express has been criticised by its largest shareholder for not properly assessing a proposed merger with rival Stagecoach.
The Cosmen family from Spain, which has an 18.5% stake in the transport group, said it had "serious concerns" about the lack of a strategy at the firm.
National Express ended talks with Stagecoach on Thursday because it felt a deal could not be done by Christmas.
It plans to raise funds by issuing more shares instead.
The group has debts of nearly £1bn.
"We have serious concerns about the absence of a well-defined strategy to address the company's broader and longer-term issues," the Cosmen family said in a statement.
"We are concerned that there has not been a sufficiently full and thorough assessment of all the available options to address the company's short and longer-term challenges."
The family was particularly concerned that Stagecoach's merger proposal had not been properly weighed up, "an option that could have addressed the fundamental financial and strategic issues facing the company".
The family also urged the board to seek independent financial and legal advice to review the company's options.
Shares in National Express rose 3.8% in early trading to 333 pence. They had fallen 12% on Thursday after walking away from the proposed tie-up with Stagecoach.