Thunderballs

Any junior spy worth their salt knows that the best invisible ink is lemon juice. Simply squeeze, dip and scribble, then hope that the recipient doesn't just discard your conspicuously blank note. Instead, they need to hold the note over a candle flame (taking care not to burn the paper) and your top-secret missive will be instantly revealed.

If you thought that invisible ink only existed in the minds of second-rate writers, who envisage clandestine catch-ups between black-coated strangers on park benches, you might be surprised to know that MI6 spent the early part of the last century investigating its uses.

According to a new report in the Telegraph, the Secret Intelligence Service experimented with a variety of ink substitutes, none of which you'd be likely to find on the shelves in Rymans. Rather bizarrely, their preferred alternative was in plentiful supply, but came with its own complications - not least, a sperm count.

That's right, the first chief of MI6 believed that semen was the best invisible ink. Of course, his name was Mansfield Cumming, so perhaps his was a biased viewpoint.

Anyone who uses a fountain pen knows the standard shaking motion involved in liberating the last few ink drops. And by the sounds of things, a similar technique would be employed for generating this particular kind of spy stationery.

The boffins at MI6 were particularly enamoured with their spunky solution because it "would not react to iodine vapour" and was "readily available'. Although it's a little more awkward than simply popping in a new ink cartridge, especially in polite company.

Unfortunately, the unnamed agent who originally recommended man-fat for scribbled subterfuge had to be transferred when he became the victim of 'jokes from other staff'. Presumably this means he was sent a lot of inter-departmental memos on scrunched up toilet paper.

The other main reason that his seminal discovery never took off, was that 'fresh supplies' were recommended, since those receiving his messages 'noticed an unusual smell'. If only he'd thought of using brie wrapping for note-paper, the world of modern espionage could have been a very different place.