Police forces across the UK are warning shoppers about a rise in “porch piracy”, an emerging criminal trend involving doorstep parcel thefts that is costing businesses and consumers millions of pounds.

In one east London borough, police have put up posters in the lobbies of flats, while in other areas forces are using social media to provide tips on how to make life more difficult for thieves.

More than 3.5m UK homes lost at least one delivery to thieves during the 12 months from July 2023 to August 2024, according to figures recently issued by the mailing equipment company Quadient.

November and December are the busiest times of year for porch pirates, according to Quadient’s figures, with Black Friday and the run-up to Christmas.

The company obtained the figures from freedom of information requests to police forces after discovering that data on parcel theft was not readily available.

It calculated that the total value of parcels stolen across the UK rose to £376m in the past year – up from £204m in 2023. However, the true figure is thought to be much higher, as only a fraction of thefts are reported to the police.

Metropolitan police safer neighbourhood teams in areas including upmarket Kingston upon Thames in south-west London have been highlighting the possibility of theft, as have police in Northwich in Cheshire and Wiltshire.

In Barking and Dagenham in east London, the Gascoigne safer neighbourhoods team has this month been putting up parcel theft crime prevention posters in apartment buildings warning: “Parcel thefts are happening in your area.”

But Quadient said a lack of data on the thefts meant customers were being left in the dark about the risk.

“Delivery companies contracted by retailers have to keep a record of delivery failures, but it’s vital that the public is also aware of the volume of thefts so they can try to mitigate the risk,” Gary Winter, the firm’s vice-president of global strategic initiatives, said. “If you live in a flat in London, for instance, there’s a one in three chance you will have a parcel stolen.”

Sportswear is three times more likely to be stolen than other goods because of branded packaging and a high resale value, Quadient found.

People who live in blocks of flats were 24% more likely to have a package stolen from outside their home than those living in a house, while shoppers aged under 34 were the most likely to report a theft.

While many parcels are stolen from outside homes by opportunist thieves, it seems unscrupulous delivery drivers are also defrauding customers.

“I’ve heard from people who have managed to capture couriers leaving parcels at doors, photographing them for ‘proof of delivery’, and then wandering off with the parcel,” said the consumer campaigner and broadcaster Martyn James. “Others have screenshots of route trackers that prove courier vans never went near their property, and I’ve seen countless examples of piles of parcels being left in public foyers in apartment blocks, despite clear warnings about doing this, only for thieves to come in suspiciously quickly and pinch the lot.”

Under consumer rights legislation, retailers are liable if an order goes astray, so they typically pick up the tab when goods are stolen.

However, if a customer nominates a “safe place” for parcels to be left, they become responsible and lose their right to a refund if the item vanishes. Some traders such as Ikea require customers to tick a box accepting this, but most shoppers are unaware that they will take the hit if a purchase is stolen from their nominated location.

Despite the potential for losses, leading high street retailers contacted by the Guardian declined to comment on the problem.

The British Retail Consortium said theft, including porch piracy, costs retailers more than £1.8bn a year.

“The run-up to Christmas means a massive increase in the number of deliveries, and porch piracy can have a huge impact on families and households as Christmas gifts disappear,” said Graham Wynn, its assistant director of business and regulation. “We urge customers to either accept deliveries in person or ensure a secure location for deliveries if they expect to be out.”

Street and neighbourhood WhatsApp and Facebook groups are striking back by sharing video doorbell footage of thieves and passing this on to the police, who are able to use it as evidence to help solve crimes.

One Guardian reader living in south London told us that eight to 10 neighbours had been collaborating with each other by sharing doorbell and CCTV footage after a gang of parcel thieves started operating in their area, and that “the police are on to it”.

The problem has been growing in other countries, too, and in New Westminster in British Columbia, Canadian police are using hi-tech “bait packages” containing tracking technology to lure thieves.

The packages are left on doorsteps and in mailboxes with the consent of homeowners, and the technology inside allows officers to track and identify thieves who attempt to steal them.