British ministers are expected to face tough questions about the decision to seize an Iranian tanker two weeks ago, without ensuring that it could protect British-owned shipping in the strait of Hormuz.

Iran seized two oil tankers – one registered in the UK, the other in Liberia, but owned by a company with offices in Glasgow – on Friday in the strait, which is the world’s busiest oil waterway, transporting as much as 17m barrels of crude per day.

Iranian forces released the Liberian-flagged ship, the Mesdar, after a couple of hours, but were still holding the Stena Impero on Friday night. Tehran is currently calling for the release Iranian tanker, Grace 1, which was detained by UK forces two weeks ago off Gibraltar.

Although there are no Iranian citizens detained in Grace 1, and no British citizens on Stena Impero, critics have already questioned whether the UK confronted Iran knowing that the Gulf waterways were not adequately policed.

Lord West, a former head of the Royal Navy, said the UK should not “pretend we’re surprised” by the Stena Impero incident. He told Sky News: “What I find extraordinary is that we knew that the Iranians would try something like this a few days ago.

“I’m absolutely amazed that we haven’t implemented some sort of control of red ensign shipping within the region whereby no tanker would go in to what is clearly a dangerous zone without an escort, and I find it bizarre that we seem to have ships doing exactly that.”

West said the British navy had “too few ships” and would find it “extremely difficult” to provide escorts to merchant vessels.

Between 15 and 30 British-flagged tankers pass through the strait every day. In the region there are seven Royal Navy vessels, accompanied by Royal Marines, for force protection in the Gulf. The British presence includes one Type 23 frigate, HMS Montrose, due to be joined and then replaced by one Type 45, HMS Duncan.

In addition, there are four mine countermeasures vessels – HMS Ledbury, HMS Blyth, HMS Brocklesby and HMS Shoreham – and a docking auxiliary, RFA Cardigan Bay. HMS Kent is due to take over from HMS Duncan later in the year.