LONDON (AFP) - Britain's Royal Mail has turned to Nasa for help after a five-year-old boy wrote in asking how much it would cost to post a letter to Mars.

But Oliver Giddings probably won't be able to send a letter to outer space as the price - £11,602.25 (S$24,631.22) - is well beyond the reach of most children's pocket money.

In its reply to Oliver, who wants to become an astronaut, the Royal Mail explained on Friday how the figure was calculated.

"Fuel is very expensive and affects the cost of sending letters around our planet," senior customer adviser Andrew Smout wrote.

"Nasa also told me that their last visit to Mars, carrying the Curiosity rover, cost about US$700 million (S$987 million).

"The spaceship itself is very small so storage is at a premium. Based on how much the spaceship weighed compared to how much it costs to get to Mars, they said that something weighing up to 100 grams would cost them approximately US$18,000 to fly to Mars."

The Royal Mail added that the price of the letter would be covered by 18,416 first-class stamps.

Oliver, from near Lytham St Annes in north-west England, wrote back to Royal Mail thanking them for their reply.

"It's very expensive to send a letter to Mars. You would need so many stamps!" he added.