An apparent firebomb attack at a DHL warehouse in Birmingham, linked to Russian-backed saboteurs, was believed to be a trial run for a US attack, according to Polish officials.

Poland’s National Prosecutor’s Office has confirmed four arrests after parcels “containing explosives” were allegedly sent via courier companies to countries including the UK.

Counter-terror police in the UK are already investigating whether Russia had any involvement after a suspicious package caught fire at a DHL warehouse in Minworth in July.

Authorities in Germany are also examining several fires thought to have been caused by incendiary devices hidden inside parcels at a warehouse in Leipzig.

Polish Prosecutor Katarzyna Calow-Jaszewska said the latest arrests were related to parcels “which spontaneously ignited or detonated during land and air transport” to EU countries and the UK.

She said the group’s goal was allegedly “to test the transfer channel for such parcels, which were ultimately to be sent to the United States of America and Canada”.

She added that four people involved in “sabotage” and “of an international nature were detained”.

On Monday, Counter Terrorism Policing said the arrests reported by Polish authorities were not carried out as part of its investigation.

It coincides with reports by the Wall Street Journal (WSJ) that the devices were “electric massagers implanted with a magnesium-based flammable substance” and “part of a wider Russian plot”.

Russia has denied involvement. A Kremlin spokesperson told the US newspaper the claims were “traditional unsubstantiated insinuations from the media”.

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A suspicious fire took hold in July at a DHL warehouse in the UK after a package arrived by air, but further details about the plane and its flight path are unknown.

Last month British police said their investigation was “being led by officers from the Met’s Counter Terrorism Command with support from colleagues from Counter Terrorism Policing West Midlands”.

A prison has been become like an “airport” with drugs being brought in by drones through holes burned in cell windows, an inmate has told inspectors.

A watchdog has warned HMP Garth, in Lancashire, which holds serious offenders, is “facing major security issues” and a “breakdown in safety and security”.

Inspectors found prisoners had been using the elements from their kettles to burn holes in their “inadequately protected” Perspex windows to allow the “entry of drones laden with contraband”, while the “smell of cannabis was rife”.

Some 63% of the men held in the category B jail who were surveyed said it was easy to get hold of drugs with one saying: “This is now an airport.”

Inspectors found prisoners were damaging their windows faster than they could be repaired with 13 cells found with holes, including five which were still occupied, on the first day of the visit.

They also said oversight and searching, including accounting for mops and brooms used to collect drugs from drones, was “weak”.

Chief inspector of prisons Charlie Taylor said: “Garth holds some very serious offenders. Although the governor had a good understanding of the many challenges the prison faced, without better support from the regional team and the prison service it will continue to be a jail of real concern.

“It is imperative that the prison service finds a way to stem the ingress of drones to reduce the supply of drugs into prisons like Garth, so they can begin to reduce violence and get men out of their cells and into a full day’s work and training.

“Staff attendance and capability will need to improve significantly and without substantial investment from the prison service, drugs will continue to flow into this troubled jail.”

In January, 400-metre restricted fly zones were introduced around all closed prisons and young offender institutions in England and Wales, while there have been more than 90 drone-related convictions since June 2016.

A 36-year-old man was arrested in the early hours of Monday after staff at HMP Liverpool spotted a drone trying to land in the prison courtyard.

The package was found to contain cannabis resin, tobacco, mobile phones notes, drill bits and SIM cards, Merseyside Police said.

At the time of the HMP Garth inspection, the training prison held 816 prisoners mostly serving long or indeterminate sentences.

The rate of assaults had soared by 45% since the last inspection, with many inmates needing protection because of drug-related debt.

Inspectors found some parts of the prison were dilapidated and new arrivals said they were being forced to pay other prisoners to get missing furniture for their cells.

The report also highlights high levels of staff sickness, insufficient training and an unwillingness to challenge prisoner rule-breaking, as well as poor staff morale.

”A prison system in crisis’

It comes weeks after a damning report found category B Winchester prison was so “dilapidated” an inmate was able to remove his own cell door.

Chief executive of the social justice charity Nacro, Campbell Robb, said the issues the latest report highlights are “symptomatic of wider crisis” across the prison system.

“HMP Garth is another example of how without significant reform, we risk perpetuating a vicious cycle of violence and hopelessness within our prisons, undermining both public safety and the potential for rehabilitation in the long-term,” he said.

A Ministry of Justice spokesperson said: “The new government inherited a prison system in crisis and reports like these demonstrate the need for robust action to get the situation back under control.

“We have zero tolerance towards violence and drugs and our security measures, such as X-ray body scanners and anti-drone no-fly zones, detect and stop drugs from entering our prisons.”

More cases of a new strain of mpox have been detected in the UK, officials say.

Last week, the UK Health Security Agency (UKHSA) said a single case of the mpox virus variant Clade 1b had been confirmed in London.

The agency has now announced a further two cases have been detected.

The two patients are household contacts of the first and are currently under specialist care at Guy’s and St Thomas’ NHS Foundation Trust in London, it added.

“The risk to the UK population remains low,” the UKHSA said.

“There has been extensive planning under way to ensure healthcare professionals are equipped and prepared to respond to any further confirmed cases.”

The UKHSA’s chief medical adviser, Professor Susan Hopkins, said: “Mpox is very infectious in households with close contact and so it is not unexpected to see further cases within the same household.

“The overall risk to the UK population remains low. We are working with partners to make sure all contacts of the cases are identified and contacted to reduce the risk of further spread.”

The first case detected in London last week was a person who had been on holiday in Africa.

They developed flu-like symptoms more than 24 hours after returning to the UK and later developed a rash which worsened in the following days.

The agency said contacts of all three cases are being followed up on and would be offered testing and vaccination as needed.

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What are the symptoms of mpox and how is it passed on?

Common symptoms of the disease include a skin rash or pus-filled lesions, which can last between two to four weeks. It can also cause fever, headaches, muscle aches, back pain, low energy and swollen lymph nodes.

Mpox, and the Clade 1b strain, is usually passed on by close physical contact, contact with infected animals, or sexual transmission.

The UK has a stock of mpox vaccines and last month announced more were being procured to support a routine immunisation programme.