Britain and Iraq have begun a "long-term partnership of equals", Prime Minister Gordon Brown declared today as the end of UK combat operations there was announced.
After talks in Downing Street with Iraqi counterpart Nouri al-Maliki, Mr Brown said: "Today we are taking steps to strengthen and deepen our relationship and to make it a long-term partnership of equals.
"Today marks the closing chapter of the combat mission in Iraq. The flag of 20 Armoured Brigade will be lowered as British combat patrols in Basra come to an end and our armed forces prepare to draw down."
Mr Brown paid tribute to the British service men and women who had lost their lives in Iraq.
"They will always be remembered for the service they have given. Our country owes them a huge debt of gratitude," he said.
He said that the memorial in Basra honouring their memory would be "preserved for ever" at the National Memorial Arboretum at Alrewas, Staffordshire.
"Today Iraq is a success story. We owe much of that to the efforts of British troops. Our mission has not always been an easy one, many have said that we would fail," he said.
"Britain can be proud of our legacy that we leave there."
Mr Maliki said his country needed investment from Britain and co-operation across many fronts, including science, the economy and business.
Mr Brown said: "We hope to sign an agreement with the Iraqi government about the future role that we can play in training and in protecting the oil supplies of Iraq.
"That will be an agreement between our two countries, rather than any new United Nations resolution.
"I gather this will go to the Iraqi parliament within the next few days and this is part of a new, deeper relationship between our two countries."
Mr Brown said he and Mr Maliki had signed a new declaration of friendship, partnership and co-operation between their two countries, setting out a new basis for relations between them.
He said negotiations would be opened on an investment promotion and protection agreement while a review of export credit guarantees would be brought forward.
"This is a clear message for companies worldwide: Iraq is open for business," he said.
Co-operation would also continue on oil exploration in the Gulf, and on education - bringing Iraqi students to the UK and increasing English language training in Iraq.
"Britain and Iraq have deep historical ties. The last six years have changed our relationship again," he said.
"Today we open the next chapter in our relations, a chapter which I preface with my deep respect and appreciation for Iraq's achievements and my gratitude to Britain's armed forces who have made such a lasting contribution."