The MP for Wellingborough who was under investigation for fraud "absolutely believed" the county's former police and crime commissioner when he insisted he was not the source of a leak, a court heard.
Former commissioner for Northamptonshire, Adam Simmonds, is accused of passing on confidential information about the criminal investigation into MP Peter Bone in 2013 and 2014.
Mr Bone was under investigation with his wife Jeanette over claims they had committed a £100,000 fraud concerning benefit payments of care home fees for his mother-in-law. The investigation was not pursued and no charges were ever brought.
The MP, who has represented Wellingborough since 2005, said Simmonds had given him "his personal assurances he had not briefed against" him which he believed.
He told a jury when the story emerged in newspapers it put "immense pressure" on his family especially as his wife had been diagnosed with breast cancer.
Simmonds, 40, is accused of breaking data protection laws by sharing details about a fraud probe involving Mr Bone with Conservative colleagues.
It is alleged Simmonds had used the information to "aid the Conservative Party" by having conversations with Northampton North MP Michael Ellis and Daventry MP Christopher Heaton-Harris.
It is also alleged he had also spoken to former Wellingborough Borough Council leader Paul Bell and the then Northampton Borough Council leader David Mackintosh about the case.
Mr Bone, giving evidence at Southwark Crown Court, said he complained about the police's handling of the case after learning of the leak from journalists.
Prosecutor Christopher Foulkes said in September 2013 Mr Bone "was contacted by a local newspaper editor who said a national journalist had been touting a story, or spreading a story, that the police were investigating the MP and his wife in relation to inheritance."
Responding, Mr Bone said: "It was incorrect and I did deny it."
The prosecutor added: "You were informed around the same time the police had contacted your solicitor to say that they had also been contacted about your case, the investigation."
Mr Bone said he came to realise details about the investigation had been leaked to the press.
Mr Foulkes said: "Was it your understanding you had become aware that others appeared to know something about the confidential investigation that was being undertaken?"
Mr Bone told the court: "That became apparent later. I was amazed to find out how many people did know but I was not aware of all of them at that time."
He added that one of the 'key moments' was when another MP told him he had "been informed by a county councillor quite considerable detail about the investigation. "That was a complete surprise."
The court heard a professional standards investigation revealed several people, including two MPs and a councillor, confirmed they had discussions with the police and crime commissioner about the case.
Mr Bone explained how he had a telephone conversation with Simmonds, who he had known for "a long time", in May 2014 about the leak.
He said Simmonds told him he had spoken to other MPs about the case but "had not given any operation detail".
Mr Bone said: "He gave me his personal assurances he had not briefed against me or spoken to the Times. He said the leak was most likely to have come from the county council.
"He said he had spoken to people about the date the CPS was likely to make a decision."
Mr Bone added he learned from the police professional standards there could have been some issues with how the case was handled by the commissioner.
He said: "Professional standards came back to me and said 'we have investigated and we think there are some issues related to data protection where the commissioner has not complied'.
"So they came to me and they explained to me they could not investigate that and that I would have to make a subsequent statement which I did."
He added that he "could not understand" how the information could have been passed to anyone else.
He said: "I have known Adam a long time and I thought he was an excellent police and crime commissioner. I have great respect for his integrity and what he says, and I'm absolutely sure when he rang me up and said he didn't leak the investigation to The Times newspaper, I absolutely believed him. He could not have done. It was a completely bogus and made up allegation that should have been dealt with a lot quicker.
"We found it all over the national newspaper. It put immense pressure not only on me but more importantly probably my family and during that period my wife developed breast cancer."
Simmonds, who lives in Leicester, denies the charges. The trial continues.