Spain’s Cosmen family, which owns 18.5% of National Express, said it had serious concerns over the group’s lack of a “well-defined strategy”.

The Cosmens - who recently pulled out of a £765 million takeover of the debt-laden group - said Stagecoach’s approach “could have addressed the fundamental financial and strategic issues” facing the business.

They said Stagecoach’s approach had been dismissed too quickly and urged the board to keep its options open.

National Express said yesterday it had ended talks and pledged to press ahead with its own fundraising plan, which could involve a cash call on shareholders of up to £500 million.

The Cosmens, which took their stake in National Express in 2005 said they remained “entirely focused” on the long-term interests of the business but wanted an independent review of the options.

They warned: “We are greatly concerned that the board risks losing further value for all shareholders by not keeping the company’s options open and we would urge the board to seek independent financial and legal advice to assist it in this review process.”

Under City takeover rules, the Cosmens - who were working on an offer in tandem with buy-out firm CVC - are currently barred from making an offer without a recommendation from the National Express board or unless another company makes a firm intention to bid.

Stagecoach had submitted a preliminary offer that would have seen National Express take up to 40% of the merged group, worth £1.7 billion.

But National Express, which recently warned on profits, judged the deal was unlikely to go ahead this year and decided to press on with a fundraising as a standalone business.

The firm is still in discussions about handing back the East Coast Main Line franchise - which it paid too much for at the height of the boom to the Government.

Its other UK rail franchises are the East Anglia service and London commuter operation c2c, while it also has a bus division.

Stagecoach - the operator of South West Trains and East Midlands Trains - had been lined up to take on National Express’s UK rail and bus business under the plans drawn up by the Cosmens and CVC.

After the Cosmen bid collapsed, Stagecoach decided to make its own offer for the National Express business.