Pub staff were fined £80 after failing the test when under-age teenagers ordered drinks.

The 15 and 16-year-old volunteers were served alcopops without having their ages checked and in the presence of plain-clothed police officers.

All pubs checked in the operation in Littlehampton on Friday had previously been warned.

A Sussex Police spokeswoman said: "If at any point the volunteers had been challenged about their age or asked for proof of identity they were under strict instructions to reveal their true age.

"However, they were not challenged and each time the member of bar staff who served the alcohol was immediately given an £80 fixed penalty notice.

"The licensees were then reported for the offence of allowing the supply of alcohol and a decision as to whether they will be prosecuted is now pending."

The spokeswoman would not reveal how many pubs were checked but warned the secret testing would continued.

She said the crackdown was designed to make licensees "take a responsible attitude on the sale of alcohol" and to warn them ahead of the new Licensing Act due to be implemented in November.

Trading standards have been undertaking test purchases in off-licences for a number of years but due to the health and safety risks of taking minors into licensed premises, Sussex Police have recently taken the lead.

Chief Inspector Brian Bracher, Arun police commander, said: "Despite the fact police have already been undertaking robust licensing visits across the district, these results have shown that a covert test purchase operation is beneficial.

"The £80 has a big impact for bar staff and, hopefully, the notices will ensure they are aware it is their personal responsibility to check for age, no one else's, and they will be the ones suffering the consequences if they don't.

"However, we will also be dealing with licensees and pub owners."

Angela Hibbin, of the Public Place Violent Crime Reduction Team, said: "The fact all premises tested served minors shows a lack of responsibility by licensees in ensuring that their staff are actively checking for proof of age and this is a problem that needs to be resolved.

"Letters were sent out prior to the operation warning licensees that it would be happening, indicating that it is only indifference that has brought about these results.

"As all the premises did sell to minors, more on-licence operations will be taking place again in the near future."

Mr Bracher said: "We have been working with licensees through the local Pubwatch schemes and, therefore, the results of the operation are very disappointing.

"I have written to all licensees across the Arun district to reinforce the message that they have a duty to the community to ensure that they are not selling alcohol to children.

'While we will do as much as possible to support them we will also enforce the law.

"On this occasion I have decided not to release the names of the premises concerned."