The watches are advertised as "boutique grade" - fakes which are of such high quality, even dealers have trouble telling them apart from the real thing.

The replicas of luxury timepieces from the likes of Audemars Piguet, Panerai and Rolex come in official-looking boxes, complete with warranty cards and papers.

And there is a booming trade in them online.

The Sunday Times has found at least four Facebook pages through which these counterfeits are being sold for between $500 and $1,500, depending on the model.

One such page, which has more than 23,000 likes, claims to have been in business for five years.

Interested buyers SMS or send a WhatsApp message to a mobile phone number included on the page to set up a deal. When this reporter posed as a prospective buyer looking for a Panerai 441 watch, which retails for more than $16,000, he was quoted $1,050.

The seller even promised a "lifetime mechanism warranty" for what was labelled a "1:1 replica".

He said: "You can enter Panerai boutique without them knowing it is (a) replica." He added that the watches were sourced from a factory in China, and claimed to be the "direct distributor" of its wares.

After payment is made via bank transfer, watches would be shipped direct from the factory to customers. There would be no need for a face-to-face meeting between the customer and him, he added. And if the watch breaks down, he has "lots of watch repair contacts".

These illegal sellers appear to be doing a thriving business. The Facebook pages are filled with reviews and pictures from customers who say the watches are "worth the price" and "look damn good".

Watch dealers here say such high quality replicas started appearing about one to two years ago.

Mr William Leong, vice-president of the Singapore Clock and Watch Trade Association, said this was when counterfeit factories started using the same machinery used by established watch-makers.

A separate website that sourced such fakes from a Chinese factory claims that "original watches were fully dissected and used as samples for replication". According to the United Nations Office on Drugs and Crime (UNODC), the counterfeit goods business is worth over US$250 billion (S$342 billion). About a tenth of fake goods seized are watches and jewellery.

Mr Alfred Png, who runs Png Watch Dealer, said while replicas have tell-tale signs that set them apart from genuine ones, these often boil down to tiny details - for instance the lustre on a screw, or slight difference in the dial colour.

A similar point was made by Mr Leong, who said the material used to make the fakes is inferior. Mr Png, who has been in the business for 38 years, said: "The finishing is generally a little bit more coarse, or the leather will be a little rough."

But sometimes it is hard to tell.

Mr Png said he knew a dealer who bought a watch supposedly from a Swiss manufacturer but which turned out to be a counterfeit. "It looked so real, only when he opened the case and looked at the engine, did he know it was a fake."

It is a crime to sell counterfeit goods. Anyone found guilty of doing so may be fined up to $100,000, jailed for up to five years, or both.

And it is not just sellers who could be guilty. Criminal lawyer Josephus Tan said: "Buying a fake item and using it is itself an infringement."