Mr Blair said: "It is a battle between fear and emotion on one hand and reason and hope on the other.

Tony Blair and Bill Clinton: pushing for a Yes vote in the referendum (3'17")

"The fear is understandable but it is important that people vote for hope."

Speaking in the grounds of Weston Park, the Shropshire countryside retreat for leaders attending the G8 summit in Birmingham, Mr Clinton asked voters to look forward.

He said: "What will happen if they vote No and what have they to lose if they vote Yes?

"I am convinced there will be a great deal of interest and investment in and partnership with Northern Ireland if this matter can be carried forward.

"The elation that will sweep the friends of Ireland in the US will be enormous. It will lead to a greater willingness to get involved in trying to help build the future.

"Voters have a win-win option."

Tony Blair: Hope for investment

Mr Clinton, describing himself as an outside observer, said: "The people for this want a better future for their children not more violence.

"The people against this still do not trust those on the other side."

Ulster Unionist leader David Trimble also stepped up the Yes campaign pressure.

David Trimble: "There are going to be continuing difficulties" (0'13")

He said: "This agreement provides a more stable environment than we have ever had before.

"We have to stop nit-picking and clinging to the Troubles as if they are a comfort blanket."

At the G8 summit on Saturday the peace process was welcomed by leaders of the eight richest industrial nations as a basis for peace and prosperity.

Eight parties reached agreement on April 10 on an accord that would see Catholics and Protestants sharing power in a Northern Ireland Assembly.

For ratification, it must be approved in a referendum on Friday in both Northern Ireland and the Republic of Ireland.