Two well-known security companies, Ambrey and Maritime Asset Security and Training (MAST), are reportedly amongst those who have replaced UK citizens with guards from other countries as tanker tensions between Iran and Britain remain high.
The move follows Iran's seizure of Stena Impero in the Strait of Hormuz. Until today the vessel and its 23 crewmembers remain held by Iran. Recently, Iran stated that they deny the UK to sail through the Strait of Hormuz, as they are not allowed to sail thorough the Strait of Gibraltar.
Ben Stewart at MAST commented
We’ve been recommending not using UK unarmed guards because of that [risk].
Moreover, the developments in the Strait are rapid, as the US is launching a coalition to better-secure foreign vessels transmitting through Hormuz. The coalition includes the UK and South Korea, whereas Australia and China are still considering their joining. In the meantime, Germany and France have already declined US's invitation, in light of potential escalation.
The tensions between US and Iran began when the former withdrew from participation in the JCPOA (Joint Comprehensive Plan of Action) and re-imposed US nuclear-related sanctions. Following this event, the tensions peaked after a number of ship attacks in the Strait.
Here you can see a timeline of the events that followed.