"That is not justice.
"Obviously, if Det Supt Fulcher has broken the rules, he should be disciplined. We have to have rules and regulations in place but that is a separate issue and should not affect the overall outcome of a potential trial.
"Any evidence, whether it proves somebody's guilt or their innocence, should be admissible in court and should always come before a jury."
David Hines, a spokesman for the National Victims' Association, said the Pace guidelines needed urgent reform.
"The rules are so stringent they are not always going to be right for an officer investigating a murder, whose hands should not be tied," he said.
"They are out of date now and need to be reviewed."
Robert Buckland, Tory MP for Swindon South, the home town of both victims, said the 1984 Act should not be immune to change.
"This tragic case, which is causing so much pain to Becky's family, illustrates the need for careful look at how the Pace code is to be applied in emergency situations such as this," he said.
"In particular, consideration should be given as to the way in which a suspect is to be informed of their rights when a second offence is disclosed during the course of an investigation.
"Since its introduction, PACE has made a huge contribution to criminal investigation, but its Codes are not and should not be immune to change in the light of experience."
David Davies, a Conservative backbench MP and serving Special Constable, said he had already tried to amend the Police and Criminal Evidence Act during the last government.
"Obviously it does need reforming," he said. "At the moment it stacks the odds in favour of the offender.
"The case of this officer who tracked down a murderer and found two bodies is only the most serious example.
"It must be right that where a suspect volunteers extra information, the police are able to act on it.
"I welcome the tough speech by David Cameron today but I would now like to see the Home Office act on it so that we don't have a set of regulations that favour the criminal over the victim - and Pace would be a good place to start."
The Home Office insisted that the Act had "stood the test of time" and was designed to protect the rights of both victims and interviewing officers.
A spokesman said: "The Act was introduced as a direct response to concerns over the conduct of the police in significant miscarriages of justice in the early 1980s and has stood the test of time ever since."
The previous government conducted a full review of the Act but no comments were made in respect of investigative interviewing.
Halliwell was arrested in an Asda car park in Swindon, read his rights, and given an "urgent interview" in the back of a police car, which was recorded.
The interview was designed to find out if Miss O'Callaghan was still alive but when he refused to answer questions, Det Supt Fulcher, who was not at the scene, had the suspect driven to nearby Barbary Castle, where he believed she may have been buried, to make a final attempt to persuade him to reveal where she was.
He was accompanied by his assistant who took notes and after a few minutes Halliwell told him: "Have you got a car - we'll go."
After showing, Det Supt Fulcher where Miss O'Callaghan was buried, 20 miles away, he asked him if he wanted "another one" and took him to the place where Becky Godden-Edwards was buried, another 30 miles away.
On arriving at the police station three and a half hours after his arrest, Halliwell was given a solicitor and made no further comment.
The judge in the case accepted that there was no suggestion that Det Supt Fulcher's questioning was "abrasive or aggressive" but ruled that the suspect "spoke when he would otherwise have stayed silent."
The Independent Police Complaints Commission is investigating Det Supt Fulcher over the breach of Pace rules as well as allegations concerning briefings he gave to journalists. He has been suspended.
However, Michael Ellis, a criminal barrister and Conservative MP who sits on the Home Affairs Select Committee, said: "Over the years judges have exercised their discretion and allowed evidence to be called even where there has been a material breach of the rules and if possible the Crown Prosecution Service should appeal against this decision in the wider interests of justice.
"I am concerned that Wiltshire police have felt it necessary to suspend this officer where he appears to have been behaving in an exemplary fashion.
"There should not be a knee jerk reaction without proper cause in circumstances where officers acted in good faith and have not tried to deceive.
"Where police officers have acted in order to achieve the proper ends of justice, their forces should take a more considered approach."
Meanwhile, Halliwell's daughter, Natasha, has urged her father to "tell the truth" about Miss Godden-Edwards's murder in order to give her family closure.
Miss Halliwell, 20, told ITV Daybreak that he had written to her from prison to apologise but that he had no explanation for his actions.
