The report found half of all DIY tools rented out by hire firms failed safety tests.
Two of the companies – Jewson and Hire Station – removed tools from their shelves after the results were revealed.
Unearthed plugs, worn and broken blades, kitchen foil used as a fuse and damaged wires were among the numerous dangerous faults.
Tools from Speedy Hire, Jewson, HireStation, Travis Perkins, Brandon Hire and Independent firms all failed tests.
All three tools from Speedy Hire had faults, while two out of four Jewson tools failed the test.
Speedy Hire managing director, business and customer excellence Martin Knott said: "We take our responsibility to health and safety extremely seriously and are grateful to Which? for bringing this to our attention. The three tools which were found to be faulty were immediately taken out of circulation. We also carried out a thorough internal investigation into the procedures at the depots in question. All tools were re-tested and we have coached all relevant staff on correct company policy.
"We are currently undertaking a national pre-hire inspection 'refresher' training programme for our entire test & run operatives. We have amended our company policy to continue to offer PPE safety kit to account trade customers, but for non account customers it is mandatory to issue this, relevant for both the task to be undertaken and the product being hired on a ‘Use or Return’ sale basis."
Travis Perkins failed two out of four with problems including a petrol hedge trimmer with a broken blade extension – the part designed to reduce the risk of injury.
In a statement the company said: "The two tips of the blade extension are broken but they are level with the blade so are not a safety threat. We are disappointed to find that the water plug was missing and it shouldn’t have been hired out. However, the tool can be used for dry cutting as well as wet cutting. We have robust systems in place to ensure tools are checked before hire."
A statement from Jewson said the company was extremely concerned by the findings.
It said: "Jewson immediately launched an internal investigation and removed the tools in question from our branches. Our investigation found that these tools fell short of our normal high standards of safety and quality and should not have been hired. As a result of this investigation we have put new measures in place, including additional safety checks, to avoid any recurrence of this incident in the future."
The HireStation tools, hired through Homebase’s telephone service, also failed two times out of four.
HSS was the only company whose tools all met the British standards and guidelines from the Institution of Engineering and Technology.
HSS key accounts manager and marketing director Fiona Perrin welcomed the investigation. She said: "We’re really serious about safety and we work hard to make sure everything we hire out is well maintained, safe and ready to use. We’re delighted that an independent investigation has recognised our hard work and that we’re outperforming the industry in an area as fundamentally important as safety."
Which? chief executive Peter Vicary Smith said: "DIY enthusiasts trust these stores to provide safe, reliable equipment, but half the tools we tested failed our rigorous tests and some could have caused serious injury. We expect higher standards from hire companies."
Some of the faults were reported to trading standards.