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Shares in Thomas Cook have plunged 30% after analysts at a bank said the travel firm's shares were "worthless".

Thomas Cook's tour operations and airline are worth £738m, but its debt is around the same "and implies zero equity value", according to Citigroup.

Citigroup's damning conclusion comes a day after Thomas Cook issued its third profit warning in less than a year and reported a £1.5bn half-year loss.

Its outlook was "significantly weaker than expected," Citigroup said.

The analysts also pointed to a warning from auditor EY on Thomas' Cook's results which warned of "material uncertainties" over the group's sale of its airline, on which a new £300m bank facility depends.

"Like the auditors we see material uncertainties around the airline sale and the new debt facilities," said Citigroup.

It also warned that the firm's recent poor performance could "unsettle consumers and drive further weakness in bookings".

Citigroup's concerns were shared by analysts at Morgan Stanley, which said its worst-case (bear) forecast was also that the firm would be worth zero. However, it said if things went well (bull case), the firm's shares could rise from their current price of around 20p a share to 50p.

"With the share price at 20p, the valuation is discounting something towards in the middle, but we think the bear case is more likely than the bull case," it said.

Thomas Cook is in the process of seeking bidders for its fleet of 105 jets as it tries to raise funds for the business. The travel firm said on Thursday that it had received "multiple bids".

The company is seeking to cut costs. It has closed 21 of its High Street stores, its currency arm Thomas Cook Money is under review and more "cost efficiencies" are planned.

It has blamed a series of problems for its profit warnings, including political unrest in holiday destinations such as Turkey, last summer's prolonged heatwave and customers delaying booking holidays due to Brexit. But it has also suffered from competition from online travel agents and low-cost airlines.

Thomas Cook declined to comment on the analysts' notes.