In October 1965, the Joint Intelligence Committee estimated that even a full trade embargo would fail to cripple Rhodesia's economy due to sanctions evasion enabled by Portuguese Mozambique and South Africa, but suggested that prolonged and severe economic pressure could induce the white electorate to overthrow the government.

In early 1966, Prime Minister Harold Wilson made statements that sanctions were working, predicting that the Rhodesian government would soon fall.

At the same time, the Foreign Office was feverishly negotiating with the Portuguese Government to participate in the sanctions, which would effectively impose a total blockade on Rhodesia.

During the first week of February 1966, it became clear that Rhodesia continued to import oil by land from Portuguese Mozambique, and Prime Minister Wilson was warned that black African states could push for more urgent sanctions and raise the matter at the UN.

Following the reports of Rhodesia defying the oil embargo by sea, the British felt pressure to take action and thus prove their commitment to sanctions.

Various British warships cruised the Mozambique Channel 20–45 miles (32–72 km) from Beira and checking on oil tankers heading for the port[citation needed].

However, in the event that permission was granted, British warships were only allowed to demand it change course in the name of its flag state, and fire a shot across the bow if it did not work.

The use of force was not authorised, and if the tanker absolutely refused to comply, the shadowing warship could take no more action and only follow it to within Mozambique's six-mile territorial limit.

As a result, the Royal Navy alone had to enforce the blockade without assistance, and tankers with oil for Rhodesia could freely dock at other Mozambican ports.

As well as lacking UN permission, the British judged it outside their capabilities to blockade other Mozambican ports, at the cost of risking relations with Portugal, a fellow member of NATO.

[8] The patrol was finally eliminated on 25 June 1975, when Mozambique gained independence from Portugal and assured Britain that it would not allow transship oil to Rhodesia.