Guilty parties in the Horizon scandal “will have to go to prison for this conspiracy of lies”, Sir Ed Davey has said.

The Lib Dem leader’s brief as consumer affairs minister in the coalition government between 2010 and 2012 included overseeing the Post Office.

Sir Ed claims he was “lied to” by the Post Office, after he met Sir Alan Bates and put the former subpostmaster’s concerns about the Horizon IT system to company executives.

Asked how he felt about being “lied to”, Sir Ed told the PA news agency: “I think if you were a subpostmaster lied to by the Post Office, if you were one of the many ministers over this whole period lied to, whether you were in the media and you were lied to, whether you were in the courts – the lawyers, the judges, the juries – who were lied to, I think all of us, particularly on behalf of the subpostmasters who suffered, I think we should be angry on their behalf.

“That’s the reason why I was campaigning with others for the inquiry, which is really trying to get to the bottom of this and hold people to account in the Post Office and in Fujitsu.

“It’s why I’m a big supporter of the police investigation because I think people will have to go to prison for this conspiracy of lies and, ultimately, I think it’s for the subpostmasters – they need compensation, they need it urgently, generous, but they also need to know the people who are responsible for this – they need to be held to account.”

The Post Office Horizon IT Inquiry is ongoing and the issue is subject to a Metropolitan Police investigation.

Sir Ed Davey performs a yoga pose on a paddleboard in the River Thames
Sir Ed Davey takes part in paddleboard yoga on the River Thames (Jonathan Brady/PA)

The faulty computer system was found to have falsely recorded shortfalls when Post Office staff used it for tasks including accounting and stocktaking.

Speaking on the Thames riverbank in Goring-on-Thames, Oxfordshire, Sir Ed said his party would push for a “duty of candour” in law to force people to tell the truth and “if they don’t they are going to go to prison”.

He has also called for an Office of the Whistleblower “so people know that there’s someone to go to if they have concerns that their organisation is lying on an industrial scale”.

It follows a series of scandals which Sir Ed listed: the Post Office Horizon IT scandal, the infected blood scandal and the Hillsborough disaster.

Sir Ed Davey wearing a life jacket topples into the River Thames from a blue paddleboard
Sir Ed Davey, left, topples off a paddleboard in the River Thames (Jonathan Brady/PA)

The party leader has come under fire for his stunts on the campaign trail, with former subpostmaster Lee Castleton accusing him of “buffoonery”.

Mr Castleton, from Bridlington in Yorkshire, lost his legal battle with the Post Office after he was found to have a £25,000 shortfall at his branch.

He previously told the BBC’s Sunday Morning With Laura Kuenssberg: “It’s really, really, really important that we trust him and trust is never going to be built by swinging around on ropes or paddleboarding in Cumbria.”

In Oxfordshire, Sir Ed participated in a paddleboard yoga session on a floating platform in the River Thames, upstream from Reading.

He said his visit had “two messages in one” – first about the health benefits of outdoor activities and secondly about sewage.

Sir Ed said: “Now fortunately, this stretch of the Thames hasn’t got too much of a problem, but if you talk to rowers who row at the world-famous Henley Regatta, they’ll tell you in parts of the Thames, if they go in, they sometimes get sick.

“So there’s a big issue on public health here, a big issue on the environment, and a big issue about enabling people to enjoy the water, whether it’s paddleboarding, rowing, swimming, whatever it is.”

The Liberal Democrat leader said in the final week of the General Election campaign, his campaigners are “going to be talking to everybody, particularly those undecided Conservatives”.

He said he is “grateful” for support from would-be Labour and Conservative voters, and added: “Some of them will have to break a habit of a lifetime, and that can be really difficult for people, but I hope when they listen to us on the economy, on the health service, on the police, on the environment, they’ll know we’re the right home for them.”