The burglar is unarmed but you have a knife. What do you do?

Chris Grayling is changing the law on self-defence at home – and the consequences could be profound. By Cahal Milmo

Aburglar lying unconscious on the floor is stabbed several times by a homeowner. It might sound like a gory bad dream, but this was the scenario repeatedly invoked over the airwaves by the Prime Minister and his Justice Secretary yesterday to persuade voters that the law is on their side.

The blood-soaked hypothesis was raised by David Cameron and Chris Grayling to describe what would not be legal under their proposal to amend the law to allow householders to overreact when tackling intruders inside their property.

But it is a measure of the enduringly emotive nature of the debate about what action can be taken against burglars that the nation's political leader felt it necessary to outline a likely homicide as an example of what would be "grossly disproportionate" – and illegal – when acting in self-defence.

Liberty, the leading civil rights group, yesterday condemned the policy as "irresponsible" and warned it appeared designed to encourage "vigilante execution".

The initiative, instantly billed as showing a shift in Conservative attitudes from "hug a hoodie" to "bash a burglar", will seek to enshrine in law the idea that a householder is entitled to overreact or take "disproportionate" action against an intruder. Current legislation only permits householders to use "reasonable force".

Using the sort of language that earned warm applause at the Conservative Party conference – and left many criminal lawyers scratching their heads – Mr Grayling said: "I think householders acting instinctively and honestly in self-defence are victims, not criminals. They should be treated that way. That's why we are going to deal with this issue once and for all."

Mr Cameron, who revealed that he had himself been the victim of several break-ins, including an incident in which the intruder also stole his car keys and used his Skoda to take away the spoils, and Mr Grayling emphasised that they wanted to legislate for the natural fear felt by burglary victims who may act excessively in the heat of a moment.

Speaking on Sky News, the Prime Minister said: "We're saying you can do anything as long as it's not grossly disproportionate. You couldn't, for instance, stab a burglar if they were already unconscious.

"But we really should be putting the law firmly on the side of the homeowner, the householder, the family, and saying 'When that burglar crosses your threshold, invades your home, threatens your family, they give up their rights'."

The pledge to amend legislation touches on one of the most politically-charged areas of law following cases such as that of Tony Martin, the Norfolk farmer who was jailed for life for murder after he shot dead a teenage burglar in 1999. After an appeal, the verdict was reduced to manslaughter and he served three years in prison.

Prosecutions for individuals tackling intruders on any premises remain extremely rare, with 11 cases between 1990 and 2005, seven involving incidents in a domestic home.

In recent years, the legal – as well as political – tide has also moved in favour of burglary victims. Britain's most senior judge, Lord Judge, the Lord Chief Justice, last month emphasised that a householder was entitled to take reasonable action to "get rid of a burglar" and assessing that was not a "paper exercise six months later". He said: "You have to put yourself in the position of the man or woman who has reacted to the presence of a burglar and has reacted with fury, with anxiety, with fear ... and who has no time for calm reflection."

But amid anxiety that legalising disproportionate violence would set a dangerous precedent, senior lawyers warned that the proposals were a political gimmick which risked muddying the law with populist sentiment.

Michael Turner QC, chairman of the Criminal Bar Association, said: "There's no concept in British law of allowing someone to use disproportionate force for good reasons. There is absolutely no need to change the law on self-defence as it stands.

"This is about making a political headline. It would be an unnecessary piece of legislation, which has real potential to be dangerous."

Shami Chakrabarti, director of Liberty, accused Mr Grayling of showing both ignorance and disrespect for the law. She said: "Terrified householders defending themselves are already protected, so this irresponsible announcement can only be designed to make people afraid or actually encourage vigilante execution."

Mr Cameron insisted he was not signalling a move towards American-style "stand your ground" laws which allow homeowners who believe they are under threat to act with lethal force. Police in Connecticut are still investigating a case last week where a teacher emerged from his home and shot dead a masked, knife-wielding intruder on his driveway, only to find that the apparent burglar was in fact his teenage son.

The toughened British legislation will nonetheless provide homeowners with some of the most extreme powers anywhere in the world.

In Australia and many European countries, an individual can only act with "reasonable and proportionate" force. By contrast, legislation in Israel allows homeowners to open fire on any trespasser who is reasonably believed to have criminal intent.