New Justice Secretary Alex Norris has been urged to reconsider the early release scheme after being appointed by Andy Burnham on Monday.

Asked about reports that the prime minister could be working on exempting rapists and child groomers from the scheme, his spokesman said: "The government's fully focused on keeping the public safe.

"We've got no option but to take decisive action to fix the crisis in our prisons. I'm not going to get into speculation over possible policy changes.

"Of course, the Prime Minister will take decisions on all manner of policy areas in the future, including how we get justice for victims."

Aileen O'Connor, chair of Thames Valley Police Federation, said: "The killers of PC Andrew Harper should be kept where they belong... behind bars.

"Police across Thames Valley - and indeed the country - will be disgusted at the prospect of these smirking lowlifes being free to roam the streets.

"These killers have never shown any remorse for their horrific and deadly actions that night.

"Andrew's wife Lissie and his whole family are serving a life sentence. The least we can expect is that his killers serve their whole sentence. However poor it already is."

She added that they will be communicating with the Police Minister, the Home Secretary and the Prime Minister.

"We will not stop. Andrew deserves nothing less," O'Connor said.

"What message does this send about the worth of a life of a police officer? The government and the parole board must reconsider any plans to release Andrew's killers early. And they must do it now. No ifs. No buts."

Cabinet minister Heidi Alexander said the early release scheme was necessary because the justice system had been on the brink of collapse when Labour came to power.

She told BBC Breakfast: "If we ran out of prison spaces, courts couldn't sentence and the police wouldn't have been able to arrest.

"And no responsible government would have allowed us to get into that situation."

Meanwhile, new defence secretary Wes Streeting has insisted there was "no ill intent" behind apparent comments about the early prisoner release scheme, which were caught on camera.

He can be heard saying to Norris that "he wants to let them all out" - appearing to reference the government's plans for early release for prisoners.