Plymouth was not authorised to use force, the tanker freely sailed into Beira, the British lobbied for UN authority to use force. The UN Security Council subsequently passed Resolution 221. However, the resolution confined the blockade to Beira and authorised only the Royal Navy to use force. 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. The resolution also empowered the British to seize the Joanna V upon its departure from Beira if it had discharged its oil cargo there. On 8 April 1967, the frigate intercepted the tanker Manuella and escorted her away from the Mozambique Channel, but when Berwick broke off for refuelling, Manuella again set course for Beira, but was intercepted by the frigate , which kept the tanker from reaching Beira. The rules of engagement were subsequently liberalised, but use of force was limited "to the very minimum", and Defence Ministry approval was required for the diversion of vessels. British warships also had to remain outside of Portugal's territorial waters. After an embarrassing incident involving the French tanker Artois, which ignored a challenge from the frigate with Minerva firing warning shots before the MoD signalled that Artois was authorised to enter Beira, the rules of engagement were further modified, allowing the use of disabling fire. Britain never managed to obtain UN authority allowing other navies to participate. 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. South Africa was also capable of transporting oil to Beira by having tankers cruise through South African and then Portuguese territorial waters, providing legal immunity from interception. As a result, Rhodesia continued to receive oil shipments. Rhodesia was also able to withstand the blockade by strictly rationing oil. In September 1966, it was estimated that Rhodesia received 220,000 gallons of oil daily, when it needed only 200,000 a day under its rationing policy. The patrol was gradually reduced in several stages. In March 1971, new Prime Minister Edward Heath allowed the Royal Navy to commit one warship at a time, rather than two. Three months later, the patrol lost its air component when the Malagasy Republic asked the Royal Air Force to eliminate its detachment at Majunga. After an overall