The offer only applied to crossings between 0015 and 0245 BST

A member of the public complained to the Advertising Standards Authority (ASA) which ruled the advert was "misleading".

The customer, from Wales, had looked into the deal only to find the maximum discount was only available on ferries leaving Dover between 0015 and 0245 BST and on tickets booked over the internet.

The ASA upheld the complaint and said the advert "did not fully make clear the nature of the offer".

'Very key restriction'

It said the limitations to the offer were "likely to disappoint customers who were unaware of the restrictions".

The adverts, placed in national newspapers, offered a 65% discount as long as the trips to France were booked by 8 May but did not say what time of day the trips had to be taken.

We'll be keeping a close eye on their adverts to make sure that they don't break the rules again Claire Forbes Advertising Standards Authority

P&O said they had been running similar offers since January and regularly placed adverts to inform customers when there were only a few days left to book for special deals.

Claire Forbes of the ASA told the BBC: "If there are special offers, I think people expect there to be some kind of small print, terms and conditions that they have to take into account.

"This key timing criteria wasn't present in the small print of the ad and I think that most customers would think it's reasonable to expect P&O to have made that clear.

"One of the key things we object to is there was a very key restriction to the offer that wasn't made clear anywhere in the ad.

'No plans to mislead'

"We've told P&O they must make sure that in future advertisements that they don't repeat the same mistake.

"They've said to us that they will take this into account and as far as we're concerned that's where the matter rests.

"But we'll be keeping a close eye on their adverts to make sure that they don't break the rules again.

A P&O spokesman said the firm understood and accepted the ruling, the company had no plans to deliberately mislead its customers and that when it got things wrong it acted quickly to put them right.

A second complaint - that the availability of promotional fares had been exaggerated - was not upheld.