WORRIED holidaymakers have been contacting Thomas Cook with fears about their holidays after the company shares were branded 'worthless' after a £1.5billion loss.

Stock market analysts Citigroup declared on Friday that the company's shares should be marked at zero and chief executive Peter Fankhauser admitted it had made operational losses of £1.4bn in six months.

Thomas Cook, the UK's oldest travel firm, is also labouring under the burden of "heavy debts" of around £1.25 billion.

Warning of a bleak outlook for the business, Citi analyst James Ainley said its troubles were likely to 'unsettle consumers and drive further weakness in bookings'.

READ MORE: Thomas Cook shares drop 40% amid bail-out bid

News of the company's problems has caused panic among some of its customers.

On the Thomas Cook Facebook page, one traveller posted: "I'm flying to Menorca six weeks today. I hope the financial situation will be settled soon."

Another said: "I just booked for next May to Turkey I really hope you're not going into liquidation."

A third wrote: "Are you going into administration? Because I have a holiday booked in October."

Justin Waite, 48, from Southampton, who hosts a podcast for private investor community Vox Markets, booked a holiday to Mexico for July with Thomas Cook and is worried about the company's uncertainty.

The father-of-two said: "I am a private investor and so I know about balance sheets and at the moment Thomas Cook have liabilities four times the size of the entire company and they are making a loss so they can't reduce this debt.

"They lost £1.4billion in the last six months.

"Luckily we have insurance but I fear there may be some who don't. I will not book another holiday with them again as I fear they are going to go bust.

"I'm sure I'm not the only person thinking this, which will make their business suffer even more."

Thomas Cook said that all its holidays are ATOL-protected meaning customers will be able to get a refund if it collapses.

The company has struggled in recent years to keep up with changing habits of customers.

Online aggregator sites, such as Chinese-owned Skyscanner, pose a particular challenge.

READ MORE: Thomas Cook shares nosedive on fresh profit warning as Brexit takes its toll

Rather than booking through a sole agent, many holiday-makers can now use such sites to scan the web for the best offers on flights and hotels.

The company, which is valued at £180m and had 22 million customers last year, put their airline business up for sale in February in a bid to raise some much-needed cash.

It has previously blamed Brexit and 'an uncertain customer environment' for its poor financial performance.

The company is currently taking drastic money-saving measures to ensure it stays afloat, including axing 150 head office roles, and closing 21 stores and 320 retail jobs.

Auditors Ernst & Young this week said there was "material uncertainty" around the sale of the firm's airline division.

However, while publishing its half-year results, the auditor said that the company, a British household name, should be able to stay afloat considering all the uncertainties it faces.

In a tweet, the firm wrote: "This announcement has no impact on future holidays or flight only bookings.

"All our holidays are fully ATOL-protected, so customers can continue to book with confidence."

Michael Hewson, chief market analyst at trading firm CMC Markets, said: "Thomas Cook's woes have gone from bad to worse after Citigroup downgraded the shares to sell with a 0p price target. Investors appear to be losing confidence in the ability of management to turn the ailing business around."

READ MORE: easyJet says £275m loss 'in line with expectations'

The travel firm dates back to 1841, when 32-year-old Thomas Cook sold a one-day rail excursion for one shilling per head from Leicester to Loughborough.

The first trip, inspired by Cook's belief that the working-class's lives could be improved by travel and education, carried 500 people.

The company now sells package holidays, operates airlines in several European countries, owns hotels in 47 countries and has a travel money and insurance branch.