The Home Secretary has been urged to intervene after Cumbria Police raided a property and arrested a third alleged "whistleblower" over leaked details of a crime commissioner's £700 trips in a chauffeur-driven limousine.
Officers carried out the search in Penrith on Thursday and a 54-year-old man is currently in custody on suspicion of perverting the course of justice.
He was held after two police workers were arrested and suspended from their jobs last week after details of how taxpayers' cash was being spent was leaked to the local press.
The details reveal that Cumbria's new police and crime commissioner, Richard Rhodes, the Conservative candidate elected last November, took two evening trips to events in a hired, chauffeur-driven Mercedes, at a cost of around £700.
Local Liberal Democrat MP Tim Farron has called the police arrests "high handed" and a threat to freedom, and another Cumbrian MP, Jamie Reed, Labour member for Copeland, has written to Home Secretary Theresa May asking her to "investigate the alleged conduct of the office of the police and crime commissioner for Cumbria".
Both police staff, civilian workers, are on police bail until May 25, and are currently suspended from work while an internal investigation continues, Cumbria Police said.
A third staff member, a man aged 59, has been interviewed by police officers on a voluntary basis and has also been suspended from work while inquiries continue.
As police carried out the search and arrest, Labour's MP for Copeland, Jamie Reed, wrote to the Home Secretary to say the police's conduct was "indefensible".
Mr Reed's letter states: "I write to ask you to investigate the alleged conduct of the office of the police and crime commissioner for Cumbria.
"Two members of staff at the Cumbria Constabulary have been arrested, and a third staff member suspended, following alleged 'whistleblowing' which - it is claimed - resulted in the expenses of the Cumbrian police and crime commissioner, Richard Rhodes, being leaked to the public.
"Today it is widely reported that these arrests followed a complaint from the police and crime commissioner's own office to the chief constable.
"If true, this is clearly indefensible. Such an act would not only damage the office of the Cumbria police and crime commissioner beyond repair, but more importantly, damage the reputation of the Cumbria Constabulary.
"As someone who has worked closely with the Cumbria Constabulary over many years, I know from personal experience that their reputation as one of the best police services in the country is both hard-won and well deserved. The men and women responsible for the reputation of the Constabulary deserve better than to have their impartiality questioned.
"As a matter of urgency, I ask that you determine the facts of this case as quickly as possible so that these matters can be resolved beyond doubt."
South Lakes' Lib Dem MP Tim Farron said: "Politicians of all colours regularly praise whistleblowers - and it is wrong to seek to silence whistleblowers in this case.
"Details of the expenses of public officials ought to be publicly available anyway, we shouldn't have to rely on leaks to find these things out.
"Most councils publish this information on a regular basis so why not the police commissioner?
In a statement, Cumbria Constabulary said: "These arrests form part of an ongoing investigation by Cumbria Constabulary which was launched after police received concerns that confidential information was leaked to the media relating to the Police and Crime Commissioner.
"Initial concerns were raised by a member of staff within the Office of the Police and Crime Commissioner (OPCC) after a member of the local media approached the OPCC for a comment on a story they intended to publish.
"Cumbria Constabulary has internal 'whistleblowing' policies and processes that support officers and staff who want to raise legitimate issues or concerns in a lawful and appropriate way.
"However, it appears that these processes have not been followed in this case and the investigation is focusing on data protection issues and the unlawful disclosure of confidential information."
The Carlisle News and Star first published a story showing that Mr Rhodes' office was charged £313 to take him and his wife to and from their home in the south of the county to Rydal Hall at Ambleside in January.
Another bill of £385 was for a trip from home and back to the Pheasant Inn at Bassenthwaite Lake in February.
Mr Rhodes' office said the trips were the only two times a private hire company was used to drive him to a night meeting or speaking engagement - and he stopped this practice once he saw the bill.
And it was only done because of the miles he covered and the long hours he was working. Other arrangements are now being made.
Stuart Edwards, chief executive of the Cumbria Office of the Police and Crime Commissioner, said in a statement: "A pivotal part of the police and crime commissioner's role in Cumbria is to meet the public and listen to their views on the future policing priorities.
"As part of gaining people's views, the commissioner undertakes a number of evening speaking engagements across the very large geographic county of Cumbria, often finishing late at night and following a busy working day.
"As a result of the long hours the commissioner was working, it was decided for personal safety reasons that support would be provided in terms of a driver for some evening functions with long and late return journeys.
"There were only two instances when a private hire company was used to drive Mr Rhodes to an evening meeting or speaking engagement and these both resulted in a travel time of between one and half hours and two hours late at night.
"When the commissioner was appraised of the cost he immediately stopped the practise of hiring drivers.
"The commissioner has personally reimbursed the full cost of the journeys.
"A review took place, with alternative arrangements now being progressed."
"To arrest these staff members is high handed, a threat to free speech and a very dangerous precedent."