Think you are letting your home to a friendly professional couple, then discover that someone else has moved in and the original tenants are nowhere to be seen? Then join the club. Unauthorised tenancies, and the problems associated with them, have become endemic in some parts of London.

In the nature of the problem, it is hard to quantify it, but the Audit Commission has estimated that, taking both the private rental sector and social housing into account, there could be as many as 160,000 fraudulent tenancies in the capital. And it is the same story in many other capital cities, with tenants illegally sub-letting their properties for short, or not so short, periods through house-sharing websites.
How can landlords remain vigilant and ensure that their properties are not occupied by complete strangers, renting them illegally as sub-tenants? Here are our top tips.

1. Read tenants the riot act at the start of their tenancy. Direct their attention to the relevant section of their contract and make it clear that you will not tolerate the property being sublet, even for short periods, without your permission. Acquaint them with the legal consequences of eviction and make sure they understand that, when it comes to unauthorised sub-lettings, the law is in your side.

2. Do due diligence on your tenants and take nothing at face value. Many London tenants who sub-let properties illegally do so deliberately, often with multiple properties, and may resort to forged documents such as references or credit records. You need to develop a sixth sense for such tricks of the trade, or talk to your letting agent for their advice.

3. Be wary of tenants who seem too good to be true. Like other confidence tricksters, serial sub-letters are masters of patter. “But of course I can pay the full deposit. I can have it in your account tomorrow morning.” “References? No problem. I have them with me. Here!” Run-of-the-mill tenants, with no nefarious purposes, are seldom so obliging.

4. If you are using a lettings agent, make sure to use one who will take proper precautions on your behalf. There will be tenants who treat sub-letting apartments as a business and tenants who think there is no harm in renting out their flat while on holiday. Old-fashioned vetting procedures are often inadequate to the scale of the problem. When in doubt, listen to your gut instinct.

5. Establish good personal relations with tenants at the outset of their tenancy. It may not always be possible, particularly if you are living abroad, but the personal touch can be vital in building trust. Tenants are less likely to take advantage of landlords who treat them in a friendly, considerate way.

6. Keep an eye on websites used for sub-letting properties in London. This is particularly important if something in your tenant’s behaviour has made you suspicious for some reason. You may get a nasty shock if you see your property advertised for letting without your permission, but it is better to nip the problem in the bud early than months after the event.

In many large cities, there are always going be unscrupulous tenants and sub-tenants trying to game the system at the expense of bona fide landlords. And the best way to combat them? Eternal vigilance.

• Contact Kay & Co estate agents for details of properties for sale ,to rent or block management in Marylebone, Bayswater, King’s Cross, Paddington, Notting Hill, Mayfair, Hyde Park, Fitzrovia, Regents Park, Paddington and The West End (020 4502 8468; Kay & Co).