Angry probation officers flooded into Parliament in such numbers yesterday that an overflow committee room had to be commandeered for their anti-privatisation lobby.

MPs who hurried from one packed room to another were greeted with boisterous applause as they pledged support for the fight to stop privatisation of 70 per cent of the probation service.

Probation union Napo general secretary Ian Lawrence warned MPs that his members were so furious about the government's dangerous proposals that industrial action was now "a racing certainty."

Mr Lawrence said he expected a positive result in the current ballot on industrial action. "It is only the third time in 101 years that Napo members have been balloted for action," he said.

Miners' MP Ian Lavery received a warm response from lobbyists as he urged them: "Never mind any agreements on privatisation. Oppose it 100 per cent of the way."

Applause broke out as he asked: "How can we even think that crime can profit private companies? It is absurd."

He delivered a stark warning of the inevitable consequences of privatisation, as jobs were cut and work piled up for those who managed to keep their jobs.

"More work, more work, more pressure, more pressure, less pay, less pay, less pension, less pension. You get more and more pressure, until everyone wants out," said Mr Lavery.

Labour shadow justice secretary Sadiq Khan spoke of the perils of putting sometimes dangerous people under the supervision of privateers.

"Every year 200,000 people, many of whom have done very bad things will be - inverted commas - looked after by the likes of G4S, Serco, Uncle Tom Cobley or who knows who."

Mr Khan added: "God forbid in a few years time when a bad person supervised by G4S does a bad thing, I do not want to have to say I told you so."

Howard League for Penal Reform chief executive Frances Crook pledged to the lobbyists that her organisation would not be bidding for any probation contracts.

She condemned the government's scheme as "a triumph of ideology and greed."