A police force has been accused of phone hacking by a former Hertfordshire detective. Derek Webb, who lives in Luton, has won the backing of his local MP Gavin Shuker in demanding an explanation from Thames Valley Police. Mr Webb is also the private investigator who worked for the News of the World tailing high profile targets for stories. His complaint is being looked at by the Investigatory Powers Tribunal (IPT). The IPT is a judicial body which is independent of government. Mr Webb, a former undercover Hertfordshire police officer, made headlines when he broke cover to reveal he worked as a private investigator for the News of the World. Complaint to police He followed royalty, celebrities and politicians, including former Norwich South MP and Home Secretary Charles Clarke and MP Tom Watson, a fierce critic of the now defunct newspaper. The man who has spent years keeping people under surveillance said he firmly believes that police hacked his mobile phone. He has spent 15 months pursuing a complaint against Thames Valley Police . In 2007 he was arrested by police after officers accessed his voicemail messages. One of the messages contained false allegations claiming he was linked to police corruption. It led to his arrest and the arrest of a serving police officer from Hertfordshire. The case against them was eventually dropped. Mr Webb said: \"I would like to know how they obtained my voicemail message when they didn\'t have possession of my phone and there were only two options. One is the official intercept communication, the second is that Thames Valley illegally hacked my phone.\" Had the police obtained a lawful interception warrant under the Regulation of Investigatory Powers Act they would have been restricted to using the voicemail information for intelligence gathering only. They could not use it as grounds for an arrest. Documents obtained by the BBC show that police contacted Orange to do a subscriber check on Mr Webb\'s telephone just 53 minutes before the voicemail they obtained for use as evidence. Detectives listened to the voicemail a month before the arrest. \'No case\' Earlier this month Mr Webb wrote to the Chief Constable to report what he believed was a crime of phone hacking and asked the police to pass the case to another force for investigation. Thames Valley Police investigated Mr Webb\'s complaints themselves nine months ago and upheld two of his complaints about poor treatment in custody and retention of his property, but not the one he made alleging phone hacking. The force wrote: \"On the balance of probabilities I find there is no case to answer. A special procedure production order was granted and the voicemails were obtained lawfully.\" Mr Webb said the order was signed by a judge five days after police had obtained the information when such an order should have been obtained in advance. Mr Webb also asks how the police knew of the existence of the messages ahead of the application. Mr Shuker, the Labour MP for Luton South, said: \"There is no clear evidence in this case that that is what Thames Valley did but it does leave a very large unanswered question, exactly how did Thames Valley Police get the core voicemail at the heart of this case when they themselves have acknowledged they got it through interrogating the phone a month after the original voicemail was left?\" In a statement Thames Valley Police said: \"Mr Webb\'s phone was confiscated on arrest as part of a criminal investigation and interrogated using the appropriate legislation. Thames Valley Police did not unlawfully access Mr Webb\'s phone.\" Mr Webb was one of the defendants in the case brought by Thames Valley Police against Milton Keynes journalist Sally Murrer and former Thames Valley detective Mark Kearney. Three years ago they were all cleared of charges in connection with the alleged leaking to the media of confidential information. The judge said the journalists\' right to protect sources had been infringed.