Why Sun4U customers must pay twice for their holidays

And the Civil Aviation Authority warned that Sun4U-booked travellers arriving overseas may find their airport transfers and booked accommodation unavailable to them.

The Birmingham-based company ceased trading on Wednesday night, with about 1,200 UK holidaymakers abroad, mostly in Spain.

Some of these are covered by the CAA-run Atol (Air Travel Operators' Licensing) scheme and will be able to carry on with their holidays and travel home.

But Sun4U also organised flight-only and accommodation-only trips which are not covered by the Atol arrangement.

Also, the CAA said that even if holidaymakers' trips are Atol-covered, "hotels and accommodation agents may require customers who are abroad on holiday to pay again for their accommodation".

The CAA said that Atol-covered holidaymakers would be able to claim this money back from the CAA.

Patricia Cambridge said her daughter was asked to pay again for her half-board holiday.

She said: "She was told that they would have to pay 438 for the self-catering holiday, plus 10 euros (8) a day if they wanted half-board.

"They'd actually booked a half-board holiday and paid Sun4U for that holiday."

The CAA advised those Atol-protected customers who find their airport transfer and accommodation has become unavailable to make their own arrangements and then send a claim to the CAA.

A CAA spokeswoman said today: "We understand there are around 1,200 people abroad and we are trying to find out just how many are Atol-protected.

"We are also trying to determine just how many people have forward bookings with Sun4U.

Kellie Bugby was due to travel to Majorca in two weeks' time with Sun4U along with 36 relatives.

She said: "We've tried the website, we've tried calling. We're still a bit lost about whether we are or aren't going. It's something we've been planning for a whole year. It's over 12,000 of money."

Ms Bugby, from Leicester, said she checked the Sun4U website three days ago and the company appeared to be still selling holidays.

She added: "They must have known that the ones they were still selling weren't going to go through. It's so unfair."

Why some holidays aren't covered

The collapse of Sun4U highlights the ultra-complicated system of protection for holidaymakers.

In most cases when a holiday company goes down the financial plughole, those abroad, and those with forward bookings, are covered.

This protection comes in the form of the Civil Aviation Authority-run Atol system or a bond provided to travel organisation Abta.

But the difficulty for travellers comes when they book a flight-only or an accommodation-only deal as these may not be financially protected.

In Sun4U's case, the company not only sold packages involving flights - which would be protected - but also sold flight-only and accommodation-only deals.

To add to the difficulties for passengers caught up in the Sun4U collapse, the Civil Aviation Authority has said that even those Atol-protected customers could find themselves having to pay twice for the same accommodation.

The CAA has also warned that those Sun4U holidaymakers who have just arrived abroad could find their airport transfers and accommodation unavailable.