A third man was arrested today on suspicion of preparing an act of terrorism after the discovery of 'non-hazardous substances' in Kensington Gardens. 

The London park was closed on April 17 after an Islamist 'terror cell' posted a video of what they claimed were drones carrying 'radioactive and cancer-causing materials' towards the nearby Israeli embassy. 

A number of discarded items in the park were examined by police and forensics, including jars containing a substance which was later found to be non-hazardous. 

The Metropolitan Police said a 31-year-old man was arrested on Wednesday morning at an address in the Potters Bar area, on suspicion of preparing an act of terrorism. He remains in custody at a London police station. 

Police said officers are also carrying out a search at the address in Potters Bar. 

It follows the two previous arrests of a 39-year-old man on April 21 at an address in Ealing under section 41 of the Terrorism Act 2000, and the arrest of a 37-year-old in Devon on suspicion of preparing terrorist acts. 

The 37-year-old man was released on bail to a date in July. The 39-year-old man was released without charge. 

The arrest today is part of an ongoing Counter Terrorism Policing investigation following the 'terror cell' scare on April 17. 

The Metropolitan Police blocked public access to Kensington Gardens on the day of the incident, as officers wearing protective hazmat suits and gas masks assessed 'a number of discarded items'. 

The Iran-linked group Harakat Ashab al-Yamin al-Islamia (HAYI) posted a video online beforehand claiming it had aimed two drones carrying 'radioactive and dangerous carcinogenic material' towards the nearby Israeli embassy. 

Although security sources were said to consider the video as 'extremely amateurish', authorities took their claims seriously and believed they may be a proxy 'commissioned' by pro-Iranian regime groups. 

A van belonging to the Metropolitan Police's chemical, biological, radiological and nuclear (CBRN) team, a fire investigation unit from the London Fire Brigade and the London Ambulance Service's hazardous area response team were present at the scene. 

Later, divers from the Metropolitan Police underwater and confined space search team arrived close to the bandstand.

In the video released by HAYI, two people dressed in white hazmat-style suits were seen handling drones adorned with its logo. Footage, which appeared to have been filmed in a local park, then showed a drone taking off into the evening sky. 

The Met Police confirmed the embassy was not attacked, but said at the time that it was probing the authenticity of the video and whether there was any link between it and the items discarded in the park. 

Harakat Ashab al-Yamin al-Islamia has claimed responsibility for a series of incidents since the start of the conflict between the US and Israel and Iran, including the arson attack on ambulances belonging to a Jewish charity.

It also claimed recent attempted arson attacks at a synagogue in Finchley and an Iranian TV station in Wembley which is critical of the regime. 

The Metropolitan Police said in a statement on Wednesday: 'CTP officers continue to work closely with Met Police colleagues in response to various incidents, which includes specialist officers providing protective security advice and support to a large number of organisations, community venues and businesses.

'Since the attack on the Hatzola ambulances in Golders Green on March 23, there have been 28 arrests linked to the various incidents and CTP investigations that have followed – including a man arrested earlier today in relation to a terrorist incident in Golders Green. 

'Of those 28 who have been arrested, eight people have been charged with arson-related offences and one person has been convicted of arson.'