Forget the picture of Rudolph, just print the postcode, says Royal Mail

Today the influx of illegible festive cards reaches a peak of 23.5 million

At around 8pm today spare a thought for Britain's unsung army of letter detectives. For it is at this moment that the handwriting sins of a calligraphically-challenged nation will be visited upon them in their millions.

On the third floor of an anonymous office building on the outskirts of Stockport, Greater Manchester, teams have been hard at work trying to decipher poorly addressed and difficult-to-read envelopes and letters. It is a task that falls to them all year round – but at Christmas it takes on Herculean proportions.

The centre's 200 workers will be asked to help direct some 23.5 million items of illegible mail to their correct location today when the UK postal system reaches its annual peak.

Royal Mail quality compliance manager, Julie Normansell, who trains new recruits, explained: "At Christmas you get a lot of people who don't write anything throughout the year. And it may be someone who doesn't have very good handwriting anyway."

During the first 11 months of the year it is estimated that handwritten letters comprise just over half of all mail. In December, this goes up to 70 per cent, including a significant number known here as "undeliverable" – the sort of thing addressed "Grandad".

Ms Normansell's advice to once-a-year correspondents is simple: "Print the postcode in nice block letters."

That means persuading people not to indulge their hidden talent for Gothic script or to decorate their Christmas card envelopes with festive pictures of reindeer or snowmen. Red and blue envelopes are not recommended, nor is writing with a metallic pen on a white envelope. All will end up here at Stockport or at two other centres in Plymouth and Stoke-on-Trent.

Each letter sent is initially scanned by a computer at a Royal Mail distribution centre. Those that cannot be instantly read by the electronic sensor are referred to one of the manual data entry centres such as Stockport, where an image of the offending missive will flash up on screen. Staff here have to work fast. Each operative must process 1,000 letters an hour.

They have just six seconds to decode the most illegible before they are fired back into the system for further inspection.

The most skilled workers get through 3,000 pieces of mail an hour. Those that keep up a rate of 6,000 keystrokes an hour are eligible for a £140 monthly bonus.

Peter Honoky, a letter detective for nine years, believes "speed, patience and staying focused" is the key. "We are accused of not delivering stuff but it is not our fault. We run a tight ship in difficult circumstances," he said.

Royal Mail has the biggest recruitment drive in Europe at Christmas. By Monday, 3,400 new workers will have been hired.