In a major policy reversal, the British government today invited African National Congress President Oliver Tambo to meet with a government minister later this week.

The invitation to Tambo for a meeting with Foreign Office Minister of State Lynda Chalker was viewed as a significant softening of Britain's position toward his black nationalist organization, which is leading the armed struggle against South Africa's white-led minority government.

Winnie Mandela, wife of jailed ANC leader Nelson Mandela, said in an interview broadcast Monday in England by Independent Television News that blacks in South Africa regard the current state of emergency as "a declaration of war," which they ultimately will win, The Associated Press reported from London.

Although the South African government, with the support of Britain and the United States, may be strong now, she declared, "the African National Congress is the future government of South Africa, and that is God's foregone conclusion."

Although Tambo met last year with members of the British Parliament, Prime Minister Margaret Thatcher had consistently refused any ministerial level contact unless the ANC agreed to renouce violence, a step that it has rejected.

Tambo last week called on blacks to prepare for war in their struggle for equality and power.

The Reagan administration's policy also is to avoid official contact with Tambo.

A Foreign Office spokesman said that if Tambo accepted the invitation, Chalker would convey the need to resolve the South African conflict through negotiations.

On a television program here today, Tambo reiterated a call for economic sanctions against South Africa, warning that failure to impose them would ensure what most people want to avoid. "It has been called a bloodbath," he said.

The British decision to meet Tambo comes 11 days after a prestigious group of Commonwealth figures issued a report praising South African black leaders, such as Tambo, and warning that only immediate economic and political pressure on the South African government would avert the worst bloodbath since World War II.

The invitation appeared to be part of a broader softening of Britain's resistence to implementing further economic and political measures against South Africa, a softening generated in part by the Commonwealth report's alarming conclusions.

In a development likely to increase pressure on Thatcher's government, the Archbishop of Canterbury's special envoy, Terry Waite, returned from South Africa today, expressing outrage at the conditions he found there.

At an airport news conference, he read a letter smuggled out of a South African prison that was written by a man he described as "an educated professional black male." The letter said that a group of prisoners were on a hunger strike.

"We are not allowed basic rights such as washing and toilet paper," the letter said. "The food is bad. When we complain, the authorities don't want to listen, and we have more rights taken away."