Now is the time for Mr Blair to prove the 'special relationship' is worth the effort

Tony Blair faces a test of strength today as he meets President George Bush for a summit at which the two leaders will strive to draw up a plan to restore order to Iraq and the Middle East.

Amid growing pressure on the Prime Minister to bring his influence to bear on Washington, he will try to persuade Mr Bush to give a greater and more formal role to the UN in Iraq and offer the Palestinians a real say in their future.

During talks and lunch at the White House, the two leaders will discuss the unravelling security situation in Iraq, where there has been a spate of kidnappings and attacks on Allied forces. They will also discuss a co-ordinated offensive on terrorism and the threat to Western cities from al-Qa'ida.

The talks come at a pivotal moment for both leaders, with elections looming for President Bush and Mr Blair and with falling public confidence in the US and Britain over their handling of Iraq.

Mr Blair is expected to try to salvage the Israel-Palestine "road-map" for peace and convince President Bush to allow the Palestinians a full say in their future.

Yesterday, the Prime Minister faced a backlash from Labour MPs after offering support to proposals by Ariel Sharon, the Israeli Prime Minister, to withdraw from Gaza but retain significant settlements in the West Bank without consulting the Palestinians.

Backbenchers said President Bush's decision to back the unilateral Israeli action demonstrated Mr Blair's lack of influence on the White House. The Prime Minister was hailed last year for persuading President Bush to sign up to the international road-map peace plan, a diplomatic success which helped convince many Labour MPs of the merits of Mr Blair's close relationship with the White House.

But yesterday, angry backbenchers accused President Bush of ignoring Britain and other international opinion.

Although the two leaders are likely to express solidarity over Iraq in public, Mr Blair is expected to reflect unease in Britain about American tactics which have been criticised as heavy-handed.

The crisis in Iraq deepened yesterday when gunmen killed an Iranian diplomat in Baghdad and an Iranian envoy went to Najaf to try to defuse the crisis over the radical cleric Muqtada Sadr. The US wants to arrest him and to disband his Army of the Mehdi militia. Khalil Naimi, the first secretary at the large Iranian embassy on the bank of the Tigris, was shot in the head in his car near the embassy. It was not clear if the assassination was linked to the Iranian mediation effort.

Mr Blair is hoping that talks with Kofi Annan, the UN secretary general, last night would allow him to convince President Bush to back a fresh UN resolution which would allow elections to be held within a year of the June hand-over deadline.

Mr Blair and Mr Bush will hold a joint press conference today where they are expected to reiterate their resolve not to abandon Iraq's people or the June deadline for restoring Iraqi sovereignty.

They are also expected to highlight their wish for peace between Israel and the Palestinians and an end to the violence that has dogged the region.

Tony Lloyd, the former Labour Foreign Office minister, said: "I hope the policy of the British Government is still to support the road-map. I hope there is an opportunity for some very straight talking by Mr Blair to Mr Bush. The President has benefited from very close support and co-operation from Mr Blair and he is entitled to be able to say some very straightforward things. The road-map is still the only show in town and [the Israeli proposals are] a severe setback for London, Europe and the Middle East."

Alice Mahon, Labour MP for Halifax, said President Bush's acceptance of the Israeli unilateral disengagement plan was "a total betrayal of the Palestinian people" and said Labour backbenchers would be looking for reassurance when Mr Blair addresses the Parliamentary Labour Party on Monday evening.

Mr Blair was also attacked by Charles Kennedy, the Liberal Democrat leader, and Michael Howard, the Tory leader, for refusing to make a statement about his talks with President Bush to the Commons.

The Prime Minister is also expected to seek assurances that the remaining four Britons being held in Guantanamo Bay will get a fair trial. He is expected to raise the issue of global warming and Washington's refusal to sign up to the Kyoto protocol on cutting carbon dioxide emissions.