By Jeremy Bowen BBC Middle East editor

The Quartet of Russia, the United States, the United Nations and the European Union has found its new Middle East envoy: Tony Blair.

Here are a few points for the former British prime minister to bear in mind as he begins his new role.

The position, up to now at least, has been quite narrowly defined.

Mr Blair is heading for a new role in the Middle East

The Quartet is not looking for a super-envoy, with the task of bringing peace to the Middle East.

Instead, the job is said to be "improving Palestinian governance".

Mr Blair will surely try to elbow himself into a more central role.

For him, surely, the super-envoy's job would be the only one worth having.

Mission creep

It is hard to believe that he would want to concentrate on things like making the Palestinian finance ministry work more effectively, and creating a legal system that means people do not have to go to their clans for justice and protection, however important all those challenges are for peoples' lives.

If he wants to make a real mark in the Middle East, Mr Blair will need to get to grips with the big, politically radioactive issues.

If you make a list of just some of them, they look like enough diplomatic work for a lifetime:

• the future of Jerusalem;
• creating a sovereign Palestinian state, setting its borders and ending 40 years of Israeli military occupation;
• the control of the main water aquifers under the West Bank;
• a solution for Palestinian refugees;
• a solution for the settlements Israel has built on occupied land.

Even some of Mr Blair's own colleagues in the British cabinet believed that he went too far towards Israel's position An uphill struggle?

And any envoy that comes to the Middle East to work on Palestinian "governance" will have to explain why the Quartet refused to deal with the government they elected, democratically, in January 2006.

That job might be easier now that the Palestinian President, Mahmoud Abbas, and his Fatah movement have isolated Hamas after its military takeover of Gaza.

Still, the rejection of the election result created a great deal of cynicism about Western-sponsored democracy among many Palestinians, including lots I have spoken to who did not vote for Hamas.

Hamstrung

The envoy might also want to telephone James Wolfensohn, former president of the World Bank, who was the Quartet's last envoy.

He left the job very unhappy after a whole series of frustrations. He complained, in effect, that he did not get the support that he was after.

His mission was hamstrung by the fact that all the members of the Quartet still had their own envoys, and foreign ministers, none of whom wanted to stand aside on one of the biggest issues in the world.

Mr Blair does, of course, have a record on the Middle East.

The Israelis think it is very positive. For them, he passed every test last summer, during their war with Lebanon.

Some say Mr Blair's closeness to the US will overshadow his neutrality

With the Americans, he made sure that Israel had diplomatic cover at the United Nations.

Although it did not manage to achieve its declared war aims, Israel was given plenty of time in which to try.

Even some of Mr Blair's own colleagues in the British cabinet believed that he went too far towards Israel's position.

For the same reasons that Israelis like Tony Blair, most Arabs I have spoken to have their doubts about him.

Palestinian diplomats at the UN have also complained that by siding consistently with the US on Middle East issues, Mr Blair scuppered any chance of a united European Union voting bloc emerging in the Security Council.

But it is not just his support of Israel that may raise concerns.

There is also his role as right hand man to Mr Bush in the invasion of Iraq, and his role in the disaster which followed it.

If Mr Blair gets the job, and goes to his first meeting with the Palestinians and tells them that he wants to create a Palestinian state, with borders that more or less follow the boundary that existed before the 1967 war and with a capital in East Jerusalem, they would probably be prepared to forget his past history, as long as he looked serious about trying.

It is easy to see why Mr Blair might want the job. Sorting out the Middle East, along with tackling climate change, is the biggest existing challenge in world diplomacy.

After all, what is the alternative for a retired, but still young and active, prime minister?

Signing a big contract for his memoirs, and making some serious money on the lecture circuit in the US?

Not such a bad choice to face.