United Kingdom Prime Minister Andy Burnham said he is confident he can revise his government's early-release policy to keep the most serious offenders in prison, including the killers of Police Constable Andrew Harper.

In a statement on Tuesday, Burnham said planned increases in prison capacity would enable tighter early-release rules without worsening overcrowding.

Changes made this year to the UK's Sentencing Act, meant certain inmates became eligible for automatic release at the halfway point of their sentences, rather than after serving two-thirds.

The early-release policy has been brought into focus by the case of Harper, who died in 2019 after being dragged behind a car as offenders fled a quad bike theft in the county of Berkshire, England.

His killers, Jessie Cole and Albert Bowers, could be released halfway through 13-year terms, a prospect that has drawn public condemnation. A petition to stop them being released from prison early had gathered more than 900,000 signatures by Tuesday.