LONDON (AP) — A former British soldier awaiting trial on terror charges who appears to have escaped from a London prison by strapping himself to the underside of a food delivery truck remained at large Thursday as police stepped up security checks across the United Kingdom amid concerns he may try to flee the country.

Opposition parties linked the escape to years of financial austerity, while Britain’s Conservative government said an independent investigation will take place “in due course” into how Daniel Abed Khalife managed to slip out of the medium-security Wandsworth Prison, which opened in 1851 during the reign of Queen Victoria.

His escape has prompted extra security checks at major transport hubs, particularly in and around the Port of Dover, the main boat crossing from England to France.

Britain’s justice secretary told lawmakers that “no stone must be left unturned in getting to the bottom of what happened” as he confirmed an “independent investigation into this incident.” Alex Chalk also said “urgent” reviews into prison categorization would be carried out as questions remained over how Khalife wasn’t being held at a maximum-security facility such as Belmarsh Prison in east London.

Khalife, 21, is accused of planting fake bombs at a military base and of violating Britain’s Official Secrets Act by gathering information “that could be useful to an enemy.” He was discharged from the British army after his arrest earlier this year and had denied the allegations. His trial is set for November.

Chalk said Khalife, who had been working in a kitchen at the prison, escaped at around 7:30 a.m. on Wednesday morning, when a vehicle that had made a delivery left.

Shortly afterward, he said, contingency plans for an unaccounted prisoner were activated and police were informed. The vehicle, he added, was subsequently stopped and searched by police after the alert was raised.

“Strapping was found underneath the vehicle which appeared to indicate that Daniel Khalife may have held onto the underside of it in order to escape,” Chalk said.

Opposition politicians have sought to pin the blame on the Conservative government, which has been in power since 2010. Many U.K. prisons, including Wandsworth, are over capacity and short of staff. The escape could hardly have come at a worse time for a government that is already scrambling to get all schools to reopen for the new academic year amid concerns over crumbling concrete.

“It simply beggars belief that a man being held on suspected terror charges was able to escape a prison by clinging to the bottom of a food delivery van,” said Shabana Mahmood, the justice spokesperson for the main opposition Labour Party. “How is such an escape even possible?”

Charlie Taylor, who scrutinizes detention facilities in England in his role as the chief inspector of prisons, said staff shortages are “the source of many problems” at Wandsworth.

Taylor said it “should be standard practice” for vehicles entering and leaving the prison to be checked and a prisoner has to earn a “certain level of trust” in order to be allowed to work in a kitchen.

“But the issue that we are particularly concerned about is there are too many prisoners in Wandsworth for the amount of staff who are there,” he said. “And that ultimately is the source of many of the problems in the jail.”

In an annual review, published in July, Wandsworth Prison was deemed to be a “serious concern.” The prison, which is in the middle of a residential area, holds around 1,600 defendants appearing at London courts and offenders due to be released in five wings.

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Jill Lawless contributed to this report.