Rhodesia's President John Wrathall, who helped to mastermind his country's battle against trade sanctions, has died in his sleep. He was 65. Mr. Wrathall, Finance Minister for 12 years until. appointed by the Premier, Mr. Ian Smith, as President in 1976, was believed to have died from a heart attack at Government House in Salisbury. He was known as the quiet man of Rhodesian politics, but was a key figure in the secret struggle against United Nations sanctions imposed after the Government seized illegal independence from Britain in 1965.

Mr. Wrathall, who was born in Lancaster, was president of the Lancashire and Yorkshire Society in Rhodesia. But he was frequently outspoken about Britain's role in initiating sanctions. Mr. Wrathall was sworn in as President on January 14, 1976, to succeed Mr. Clifford Dupont, who died of cancer last June. He was the country's second President following the 1969 Constitution which made Rhodesia a republic and severed the remaining ties to the British Crown.

He stood to be the last white man in the titular Presidency. Under the March 3 Salisbury majority rule agreement, reached by Mr. Smith and three black nationalists, the country is due to have a black President and black Prime Minister after December 31. The white minority handover was generally recognised as having been forced by the combined impact of a six year-old guerrilla war and effects of the trade sanctions which Mr. Wrathall fought for so long.

Lt.-Col. Henry Everard, a retired Rhodesia Railways commissioner who has deputised frequently for Mr. Wrathall, was expected to become Acting President. The Supreme Executive Council may then appoint a successor, if it is thought necessary in the four months remaining before majority rule.

Mr. Wrathall was educated at Lancaster Royal Grammar School and qualified as a chartered accountant before coming to Rhodesia in 1936. He never forgot his Lancashire background. He spent ten years in Rhodesia's income tax department. With the winds of change sweeping through Africa in the early sixties, Mr. Wrathall became a founder of the Right-Wing Rhodesian Front party and entered Parliament for Bulawayo in 1962.

He entered Winston Field's Cabinet as Minister for African Education taking on responsibility for Health and European Education when the old Central African Federation broke up at the end of 1963. In April, 1964 ho became Finance Minister, succeeding Mr. Smith who had become Premier.

Mr. Wrathall signed Rhodesia's UDI in 1965 and became Deputy Prime Minister in 1966. In 1975, he presented his 12th consecutive Budget as Rhodesia's longest-serving Finance Minister. He was recognised as a moderating influence in Mr. Smith's Cabinet. He struck a low key on racial issues with a paternalistic approach towards the political, economic and social advancement of Rhodesia's 6.8 million blacks. Before becoming President, Mr. Wrathall was one of Mr. Smith's aides during many rounds of abortive constitutional settlement talks with British and black nationalist leaders.