Tickets for the festival sold out in record time

Their inquiry follows a complaint that a pair of tickets for the music festival was not delivered to a customer in Bathgate, West Lothian.

A spokesman said the security of mail was of "paramount importance".

In May, postal workers suspected of stealing tickets for Radio 1's Big Weekend concert in Dundee were suspended and later resigned.

Meanwhile, as the festival gears up to take place at Balado in Kinross on 8 and 9 July, T in the Park bosses said tickets being sold for profit on the internet would be confiscated.

Commenting on the missing ticket claim, the Royal Mail spokesman said that delaying or tampering with mail was a criminal offence any member of staff or any other person involved in such activities was likely to be prosecuted.

He added: "Anyone who seeks to destroy the excellent reputation of Royal Mail by delaying or tampering with mail is playing directly into the hands of our competitors, as Royal Mail is now in a highly competitive market."

Tickets cancelled

T in the Park this year includes performances from The Who, the Red Hot Chili Peppers, Franz Ferdinand and The Strokes.

Geoff Ellis, of festival organisers DF Concerts, said the company had to take its own action against touts selling tickets on eBay for double their worth after being offered no support by the internet auction site.

"We have decided to fight against these online touts ourselves by cancelling their tickets," he said.

"We hope that this will prevent people from buying tickets to sell on at a profit in the future and that it will also protect genuine fans from being ripped off and left disappointed and out of pocket - as is very often the case."