Analysis By Brian Wheeler BBC News politics reporter

Why has the British government singled out Africa and climate change as the key priorities for this week's G8 summit?

Mr Blair visits a school in Ethiopia

The answer might seem obvious, given the scale and importance of the two issues to the future of the planet.

They tend to be near the top of the agenda at most global gatherings.

But for Tony Blair and Gordon Brown there are other factors - both personal and political - at play.

For Mr Blair, Gleneagles is a chance to go down in international history for something other than Iraq - and allow him to make good on the promises he made to Labour voters during the general election.

Mr Blair came face-to-face with the British public's anger over Iraq in televised debates, during what he reportedly found to be a gruelling campaign.

Private obsession

But whenever the going got tough, he was able to point to Gleneagles and his debt relief crusade as evidence of good intent.

From a very, very early age, you were hearing the tragedies and tribulations of Africa Gordon Brown

The G8 summit and Britain's presidency of the EU were presented by Labour as a unique coming together of events - a once in a lifetime opportunity to end global poverty.

Middle class voters still angry about the war were urged not to put all this at risk by deserting Mr Blair at the ballot box.

With the celebrity-backed Make Poverty History campaign capturing the public's imagination, the pressure is clearly on Mr Blair to deliver.

But even his harshest critics would admit his interest in Africa runs deeper than electoral expediency - or a desire to hang out with ageing rock stars.

The continent's plight, which he famously described as a "scar on the conscience of the world", has been one of the prime minister's private obsessions for some time.

Christian faith

He even told an audience of young people on MTV it was his passion for Africa that first sparked his interest in politics 30 years ago.

Some commentators have pointed to the fact that Mr Blair's father, Leo, a law lecturer, visited Sierra Leone university several times in the late 1960s, returning with tales of the wonderful people he had met there.

There has also been speculation about the influence of Mr Blair's Christian faith or the fact that he had African friends at university.

More recently, Mr Blair is also said to have been influenced by his admiration for Nelson Mandela, who showed him that change was possible in Africa.

Whatever the case, the relief of poverty in Africa is central to Mr Blair's vision of a post 11 September world.

Terrorism

He believes in globalisation as a potential force for good, and in the need, in an increasingly borderless world, to engage with, rather than turn your back on, neighbouring countries and continents.

Mr Blair admires Nelson Mandela

This vision has been derided in some quarters, with Mr Blair lampooned as a latterday Christian missionary attempting to bring Western values to Africa.

But Mr Blair believes passionately that global security depends on not allowing Africa, or any other continent, to "fall behind" the rest of the world and become a breeding ground for terrorism.

Even on climate change, an issue which has had less emphasis placed on it ahead of the summit amid concerns of a rift with the US, Mr Blair sees a security dimension.

At the launch of 2003's energy white paper, he described global poverty and climate change as "just as devastating in their potential impact" as weapons of mass destruction and terrorism.

"There can be no genuine security if the planet is ravaged by climate change," Mr Blair said.

Missionaries

Gordon Brown shares Mr Blair's commitment to free trade and justice for Africa and action on climate change.

He has tried to link the two issues by suggesting part of the existing tax levied on air travel, one of the fastest growing sources of greenhouse gases, should be pumped into debt relief.

He has been less vocal, in public at least, about the implications for global security, preferring to concentrate on moral and humanitarian issues.

But like Mr Blair, he can trace his interest in Africa back to his youth, when missionaries visited his father's church in Kirkcaldy, Fife.

"My father was a Church of Scotland minister and there were many contacts between the Church of Scotland and Africa," Mr Brown told the Independent earlier this year while on a fact-finding mission to Africa.

"We repeatedly heard the stories of people coming back from Africa including this area, Malawi and Kenya, telling us what needed to be done.

"I think from a very, very early age, you were hearing the tragedies and tribulations of Africa, but also the fondness of people for the continent."

Mr Brown's wife, Sarah, spent her first seven years in Dar-Es-Salaam, the capital of Tanzania, where she attended the international school.

Mr Brown has only visited Africa on one occasion, for a four-day fact finding mission, and critics have accused him of over-simplifying the continent's problems and taking a neo-colonial attitude to the issue of debt relief.

But like Mr Blair, he has perhaps scented an opportunity to make a permanent mark on history which, after eight years of grappling with the minutiae of UK public service reform, must be hard to resist, whatever the risk of failure.