Diana fountain in full flow

From a bird's-eye view, it resembles a running track. But it is in fact the Princess Diana memorial fountain, seen in action for the first time. And as the water began to flow, the workmen who have laboured to complete the £3 million project in Hyde Park on time celebrated by going for a paddle.

For almost seven years the memorial's progress has been beset by bitter infighting, hapless planning, fierce criticism, high-level confusion and sheer inactivity.

Now, at last, Diana's "moat without a castle" is near completion - so close, in fact, that workmen have released water into its impressive stone channels.

As our exclusive pictures show, what was meant to be a test-run and search for leaks turned into an impromptu paddling session.

The hard-hatted team cooled off in the clear, fast-running water and took photographs of each other to mark the historic moment.

One workman said: "Right now, everything is on schedule - we're all very happy with the way it's gone."

The official opening this summer will come as a relief to the Government, which has been criticised for allowing the commission and construction of the memorial to take more than six years.

More significantly, royal commentators predict the opening of the fountain will mark an end to the rift that has divided the Windsors and the Spencers since Diana's death in August 1997.

It has been predicted that the Queen will use the ceremony to bring about a rapprochement in what would be the first official public engagement involving both families since Diana's funeral, when Earl Spencer attacked Prince Charles from the pulpit of Westminster Abbey.

The fountain was chosen by the Diana Memorial Committee from 10,000 proposals. American architect Kathryn Gustafson's design won the final vote after a creation by British sculptor Anish Kapoor was vetoed by committee chairwoman Rosa Monckton.

Although a children's playground has been built in the Princess's memory, the fountain will be the major national monument to Diana. The 260ft by 150ft oval holds two streams of water, one slow and smooth, one fast and choppy, which converge in a dish-shaped pool.

Ms Gustafson has described her design as "historic and formal, contemporary and relaxed". As the workmen have shown, it is perfect for paddling.