John Sweeney, 48, of Eaton Road, Duston, was jailed for two-and-a-half years at Northampton Crown Court in May after admitting two counts of theft.

He appealed against the term yesterday, but was told by top judges, Mr Justice Butterfield and Sir Geoffrey Grigson, that the seriousness of his offences warranted the tough sentence.

Giving the judgment, Mr Justice Butterfield told London's Criminal Appeal Court that Sweeney had been guilty of a serious breach of the trust placed in him by his employers of 20 years.

Between November 2007 and October 2008, the Post Office discovered that 38 special delivery packages containing foreign currency had gone missing, with a total value of £53,682.

Covert surveillance cameras were installed in the special delivery lock-up at Great Billing sorting office, pinpointing Sweeney, who was already a suspect, as the thief.

Investigations showed that he had been present on all occasions when packages went missing.

When challenged, he admitted immediately that he had been taking packages randomly, exchanging the currency for sterling and paying it into his bank account in small amounts.

He said he regretted what he had done, explaining that he had an addictive personality and had spent the cash gambling at poker and on his house.

Sentencing him at the Crown Court, Judge Richard Bray described the thefts as "very serious" and said the sentence had to be tough enough to deter other postal workers to do the same.

But his lawyers argued before the two appeal judges yesterday that the resulting sentence was "manifestly excessive" and should be reduced.

Dismissing the appeal and upholding the sentence, Mr Justice Butterfield said: "This form of offending strikes at the heart of the nation's communications system.

"The public uses the system in confident expectation that post that is sent will be received, and the deterrent aspect of the sentence which the learned sentencing judge averred to is clearly an appropriate consideration.

"This was a severe sentence, at or close to the top of the range for this sort of offending, but not, in the judgment of the court, manifestly excessive."