Thousands of probation staff have started a 24-hour strike over Government plans to privatise the service.

Around 7,500 members of the National Association of Probation Officers (Napo) have walked out of offices across England and Wales in only the fourth strike in the union's 101-year history.

The union previously registered a trade dispute over Justice Secretary Chris Grayling's proposals to transfer most of the service to private firms such as G4S and Serco.

The action comes amid worsening industrial relations across the country as firefighters, post office staff and university workers have all been on strike.

Catriona Reeves, probation officer with Thames Valley Probation, said she chose to strike because she does not want public protection put in the hands of G4S and Serco.

Ms Reeves, 39, who lives in Newbury, Berkshire, said: "G4S and Serco will almost certainly win all the 20-odd contracts.

"I don't want to work for either G4S or Serco but, more importantly, I don't think that public protection should be put in their hands.

"The Government says that this will allow more voluntary and charity organisations to get involved with offenders. No bad thing.

"But why is it considered that G4S and Serco will be better at sub-contracting to the third sector than existing probation trusts?"

She went on: "Maybe you think 'Well, maybe Probation are doing a rubbish job'. But we're not. Each trust was graded between good and excellent last year.

"And the figures that the Government quote about high rates of reoffending among those sentenced to less than 12 months in prison isn't of our doing, as they are currently released without licence."

The married mother-of-two added: "I became a probation officer because I felt that I had the skills to work with some of the most challenging people in society, engage with them, manage and reduce their risk, and hopefully do my bit to protect the public.

"And I want to be able to continue to do this as part of an organisation with an internationally respected track record for doing so."

Napo general secretary Ian Lawrence, who is joining a march in Brighton, said he was confident of a strong display of support for industrial action.

He said: "We regret any impact on our client base. Our action is designed to highlight the dangers of Chris Grayling's plans for the Transforming Rehabilitation agenda.

"We believe as well as being a threat to our members' terms and conditions, there is a danger to the community with using unproven providers."

Turning to the possibility of G4S and Serco taking on contracts, Mr Lawrence said: "We don't think they're fit for purpose in any event. We believe they should be suspended from any probation work - especially as they are now being investigated by the Serious Fraud Office."

Mr Lawrence said he expects the action to disrupt the normal day-to-day running of probation services.

"It could cause court hearings to be adjourned, it could cause client appointments not to be met. We're cautious that any action we take now and in the future, we're about minimising the impact on our client base."

A package of £450 million-worth of contracts has been offered to private and voluntary sector organisations, covering the supervision of 225,000 low and medium-risk offenders each year on a payment-by-results basis.

Napo previously claimed negotiations with the Ministry of Justice (MoJ) over its Transforming Rehabilitation reforms had been ''seriously compromised'' as a result of the department's ''interference'' in the consultation on the proposals.

Three probation trust chairs wrote to the Justice Secretary last week to voice their concerns about his plans and the possible risk of harm to the public if they go ahead.

Contracts are to be split across 20 English regions and one Welsh region, while the National Probation Service (NPS), a new public sector organisation, will be formed to deal with the rehabilitation of 31,000 high-risk offenders each year.

More than 700 organisations from across the world have expressed interest in the contracts, the MoJ said, including hundreds of British firms.

A Government-wide review is being conducted of all contracts held by Serco and G4S, two of the country's biggest private providers of public services.

The audit, triggered by allegations that both firms had overcharged the Government for criminal-tagging contracts, prompted calls for the MoJ to abandon its plans to privatise the Probation and Prison Services.

But it emerged that Mr Grayling intended to allow Serco and G4S to bid for the Probation Service - though the firms will not be awarded anything until the Government's audit is completed.

Yesterday, the Serious Fraud Office confirmed it had launched a criminal investigation into the allegations.

Napo has called for the proposals to be tested and claims recent reports from America, where some states have already outsourced their probation service, suggest there are concerns about how it operates.

Justice Minister Jeremy Wright said: "It is disappointing Napo has chosen to strike when we are making positive progress in meaningful discussions with them and other relevant trade unions as we transfer to the new arrangements.

"This is a strike in favour of the status quo, which is high reoffending rates and no support for 50,000 short-sentenced offenders each year who are currently released without any supervision and go on to commit so much crime in our communities.

"We have well-established contingency plans to deal with any potential action. We will continue to support staff and engage with unions as our important reforms move forwards."

Anna Clifford, 30, a probation officer working in the Westminster area, joined a picket by Napo members outside Westminster Magistrates' Court, one of the most high-profile courts in central London.

"I believe that the proposed changes won't protect people in the long term and it will allow in untested companies to supervise offenders - basically prioritising profit over public safety," she said.

"I believe that the Probation Service is a great institution within the British justice system and it is something that we cannot lose."

Jennifer King, 38, a probation officer who joined the same picket, said: "They are saying they are going to privatise it but there is no detail as to what that actually means and what it looks like."