What do you imagine the most expensive watch in the world looks like?

If you conjured up an image of a diamond-encrusted timepiece dripping in gold and rubies, the reality is a little less spectacular.

However, the Patek Philippe 1518, which sold for a record-breaking £9million, is still beautiful, exquisitely made – and incredibly rare.

Made in 1943, the Swiss wristwatch is just one of four known examples of its kind.

The incredibly rare Patek Philippe 1518 sold for a record-breaking £9million

What makes it unusual is that it is made of stainless steel. Although the 1518 was also made in rose and yellow gold, the watchmaker did not routinely make steel timepieces until the 1970s.

Before then, steel watches were only ever ordered by professionals such as doctors or engineers who wanted prestigious pieces that could withstand day-to-day wear.

The highly sought after chronograph, which features moon phases and a perpetual calendar, was bought by an anonymous buyer for more than triple its estimated price.

It had been kept in a private collection and had never appeared at auction before. The other three known examples are also held by private owners.

Its sale at an auction house in Geneva, Switzerland, broke records for the most expensive wristwatch in history, previously held by another Patek Philippe sold a year earlier for £5.8million.

Aurel Bacs, a senior consultant at Phillips auction house, which sold the watch, said more than 400 people were in the room during the tense 13-minute bidding process. It was sold last November in an auction that saw buyers from six continents pay a total of £22.3million to purchase 177 different luxury watches.

Paul Boutros, head of watches for the Americas for Phillips Bacs and Russo, said: 'I have waited a lifetime to see a 1518 in stainless steel.'

Patek Philippe was founded in 1851, and still designs watches today. The Swiss manufacturer creates complicated mechanical pieces, considered to be among the most prestigious in the world.

Past owners of their watches have included Queen Victoria, Prince Albert and Pope Pius IX.