Owners of holiday homes and short-term rental properties in the south-west of England are being urged to check that their houses and flats are not being used as brothels.

Avon and Somerset police said they were investigating 50 suspected brothels in residential settings such as holiday flats. The force also said almost half of the intelligence reports about human trafficking and modern slavery it received in August centred on the exploitation of women and girls in off-street prostitution.

Officers say brothels are being set up not only in Bristol and other urban centres but in the countryside. There have been reports of brothels being established close to the vast building site on the Somerset coast where the Hinkley Point C nuclear power station is taking shape.

Sgt Emma Slade, who has responsibility for policing prostitution in Avon and Somerset, said: “Vulnerable women are being enslaved and exploited for sex within pop-up brothels. They are isolated and suffer terrible abuse. Many of the women are recruited and trafficked on false promises of legitimate work but find themselves in a very different circumstance.”

Slade said that when police carried out safeguarding visits to brothels, they often found women from the far east and eastern Europe who spoke no English and did not know the emergency services phone number.

As of September, the force had identified and was investigating 76 suspected brothels, 26 of which were in commercial premises such as massage parlours and the rest in residential properties.

Slade said officers relied on landlords, property owners, hosts and neighbours to report suspicious activity. “We would encourage property hosts or nearby residents to remain vigilant and report any activity that appears unusual or suspicious at short-term rental premises. We rely on the intelligence that we receive from the public and even the smallest piece of information can help us to build a picture,” she said.

Operating from short-term and holiday rental properties instead of a permanent address makes it easier for those exploiting the girls and women to avoid detection. The properties can be easily booked online without the need for large deposits or for the tenants to meet the hosts during the stay.

Supt Mark Edgington, the force’s lead for modern slavery, said: “Pop-up brothels in short-term rental properties are increasingly being reported across the UK and it is important that property hosts are able to recognise the signs of potential modern slavery and sexual exploitation.

“Many hosts are completely unaware their properties could be used for this or the signs they should be looking out for. If using an online booking service, we would always suggest that hosts check the profile of anyone requesting to book their property and read the reviews from other hosts.

“Some online services will also allow hosts to require that all guests provide identification before they are able to book. This can give some assurance that the tenant is really who they say they are.”

The police said some of the signs to look out for were:

Multiple women staying under one roof or a large number of women regularly being brought to and taken from the property.

Different men calling at the property (both day and night) and staying for only a short period of time.

Windows being blacked out or curtains left constantly closed.

A sudden increase in residential waste at a property, or more waste than would be expected for the number of tenants.

Occupants appearing malnourished, unkempt or withdrawn.

Two years ago Devon and Cornwall police issued a similar warning to owners of holiday lets to beware of sex workers using their premises as pop-up brothels.

The force said that in one resort alone, Newquay, they had discovered 14 such establishments in five months and believed more had operated but had not been discovered.