Stalkers are monitoring their victims’ movements and conversations using bugging and tracking devices as well as spyware and covert phone apps easily bought online, victims’ groups have warned.

These devices and software allow stalkers to track their victims, listen to their phone calls, or turn on their computer’s camera remotely to watch them – and are sold so cheaply on popular websites and stores that stalking is becoming “legitimised”, the groups have warned..

Tracking devices and technology hacking were used in more than 200 stalking cases reported to the National Stalking Helpline last year, though the real number is likely to be much higher due to the covert nature of the devices and software.

“[The devices] are really easy to get, they’re really easy to use,” said Clare Elcombe Webber, the manager of the helpline, which is run by the Suzy Lamplugh Trust. “I think for some stalkers it really legitimises what they’re doing... The message it sends to victims is there are all these technological advancements that help your stalker, but not you.”

Among the cheap tools that stalkers buy are bugging and GPS tracking devices, which are often disguised as common household items and are easily purchased online.

“We see this regularly... they put in listening devices or video devices in the house or tracking devices on the car and you can buy all of that on Amazon,” said Jennifer Perry, chief executive of Digital-Trust.

One woman supported by the Suzy Lamplugh Trust had a listening device that looked like a USB stick placed in her handbag by her stalker so he could listen in on her conversations.

Another woman who was being stalked by her ex-partner began receiving texts from him with details of the bedtime stories that she told her children. He had bought an extension lead that contained a listening device and installed it in the children’s bedroom so he could listen in on conversations in the house.

These USB listening devices were found for sale on Amazon and eBay for less than £20. The extension lead bugs can be purchased online for between £100 and £300 and similar items – multi-plug adapters containing a listening device – are available for sale on Amazon and eBay.

After the Guardian alerted eBay to the existence of the USB and plug listening devices, the items were removed and a spokesperson for eBay said: “This type of item is banned from eBay’s UK platform and any listings will be removed.” Amazon declined to comment.

Profile Stalking: how do I protect myself from digital surveillance? Show If your stalker knows more than they should, they could be bugging you Jennifer Perry, the CEO of Digital Trust, says a key way for victims to protect themselves from surveillance is to know it is possible. She says victims often assume their stalker knows a lot about their life because someone has betrayed them, but they could instead be under electronic surveillance. Get your phone checked by a professional "We’re always asking the question: has your stalker ever had access to your phone, and if they say yes, we always advise people to turn it off and get it checked by a professional, factory reset it, wipe everything," says Clare Elcombe Webber, the manager of the National Stalking Helpline. "If there’s any doubt at all about whether a device is safe, we generally advise people to get rid of it." Set up a new email address from a clean computer If you fear your stalker might have access to your email, Digital Trust advises you to set up a new address. If your stalker has downloaded spyware on to your computer or phone that allows them to track your keystrokes then they will be able to see any new email and password you set up, so set up the new account using a computer you know your stalker has not had access to, and only access it from these clean computers. Protect your computer If you think your computer has spyware on it, take it to a professional for checking, and download good anti-virus software. If in any doubt about its safety, get a new one. Buy a bug detector (or a cheap transistor radio) You can buy bug detectors online which will allow you to sweep your house, car and possessions for listening or tracking devices. Perry says that if you can't afford one of these, a cheap transistor radio can work as well by picking up interference. Get help and report it This kind of surveillance is illegal. Report your experiences to the police and seek help from organisations like the Suzy Lamplugh Trust or Paladin. But make sure it is safe to do so - if you think your phone is bugged, use another one; if you think your movements are being tracked through your phone, a car tracker or another GPS device, use another form of transport to go to the police station, and leave your phone behind.

Between December 2016 and November 2017, the National Stalking Helpline received 4,337 calls or emails. 77% of the victims who contacted them were female and 74% of stalkers were male. In 56% of cases the perpetrator was an ex-partner.

In roughly 130 cases reported to the National Stalking helpline last year, stalkers had downloaded spyware onto a victim’s computer or a covert spying app onto their phone.

Spyware on a laptop can allow stalkers to track their victim’s keystrokes or be used to turn on the camera of a computer and watch the person through it.

“We had one client who went home, her laptop was on, she had a shower, she then got a message from her stalker saying ‘Did you have a nice shower?’” said Elcombe Webber. “It’s that kind of invasion and not knowing how that person is able to see you. Are they outside? Are they in the house? It’s really really frightening.”

“Installing spyware without someone’s knowledge, which is generally the gist of spyware, will normally be a misuse of the Computer Act of 1990, which is a criminal offence,” said Iain Wilson, partner at law firm Brett Wilson LLP.

Spyware apps allow stalkers to monitor their victim’s phone use, including listening to phone calls, reading messages and emails, tracking GPS locations, viewing browsing history, photos and social media.

These apps are often covert and can run in the background of a phone without being open. Once installed, many of these apps can be deleted from the phone’s homepage, making them very difficult to detect.

These apps are often marketed as tools for parents to monitor their children’s phone activities. However, one popular app, which the Guardian has chosen not to name, markets itself as “software that can ... track your wife’s phone, anytime and from anywhere”. The app was removed from the iTunes store after Apple was contacted by the Guardian.

In the year to December 2016, the police recorded 4,613 stalking offences, but the National Crime Survey found that in the year to March 2015, 4.9% of women and 2.4% of men aged 16 to 59, or 1.1 million people, said they had experienced stalking, which suggests the crime is being dramatically underreported or not recorded accurately by police.

Wilson said the law has not moved quickly enough to catch up with the use of technology by stalkers and that there was a need for more police officers and CPS staff who “understand how the internet works” and can take on the work of investigating and prosecuting these crimes.