Royal Mail customers pay 28p for a first class stamp, 7p more than the price of a second class stamp, in the hope that the item will reach the recipient the next day.

BBC: NEWS | Business | Use second class, says Postwatch

The answer, as Dr Evans shows in the Proceedings of the Royal Society, is that sneaks are first-class in one particular department.

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Other parts of Wales made it into the Royal Mail's top 20 best areas - in mid Wales, more than 92% of first class letters are delivered on time.

BBC: NEWS | UK | Wales | Welsh cities among worst for mail

But in response, a Royal Mail spokesman told the BBC that any suggestion that the second class post was more efficient than the first was "completely incorrect".

BBC: NEWS | Business | Use second class, says Postwatch