British ministers fear that President Robert Mugabe's latest crackdown, which has cost 400,000 people their homes and livelihoods, could make it harder to convince the world's richest nations to agree to a "step change" in their support for Africa.
In a further setback for Mr Blair, the G8 summit may stop short of condemning events in Zimbabwe in its formal communiqué. Although Mr Blair and President George Bush will strongly criticise the actions of President Mugabe in their press conferences at Gleaneagles, Russia is expected to oppose any tough criticism being included in the G8 declaration.
Instead, Britain is pinning its hopes on the UN Security Council passing a strongly worded resolution on Zimbabwe after a report by the UN's special envoy, Anna Tibaijuka, who is visiting the country.
The British Government is confident of a deal to cancel billions of pounds of debt by African countries. But Mr Blair's hopes of raising direct aid to the continent to $25bn (£13.8bn) a year, as recommended by the Commission for Africa he set up, could be harder to achieve.
The United States, which has always demanded that higher aid payments be matched by better governance, may be reluctant to help Africa at a time when other African nations are pointedly refusing to condemn Zimbabwe. One British source said: "There is a danger that some people will argue that Africa has got to do more to put its own house in order. There are fears that Zimbabwe will cast a shadow over the summit. The timing is not exactly helpful."
Thabo Mbeki, the South African President, will be at Gleneagles and is expected to come under strong pressure from Britain and America to "come off the fence" by denouncing developments in neighbouring Zimbabwe.
Yesterday Mr Blair hinted at his private fears that progress on Africa at the summit could be constrained by events in Zimbabwe. Speaking at Prime Minister's Questions, he said: "We are going to the G8 to try and make the case for helping poverty in Africa. There is no doubt at all that it is harder to make that case while abuses of governance and corruption occur in African countries."
Mr Blair said he would continue to tell President Mbeki that he should speak out over what he called "the appalling cases of torture and abuse of human rights that occur on a daily basis in Zimbabwe".
He added: "We have to make sure that African countries realise the deep responsibility there is to sort this out themselves."
Cardinal Cormac Murphy-O'Connor, the Roman Catholic Archbishop of Westminster, joined calls for a moratorium on deporting failed asylum-seekers to Zimbabwe. He said such deportations would be "a gross injustice".
Despite growing signs that an unofficial suspension on returning people to Zimbabwe has been imposed, Downing Street denied that the Government had made a U-turn.