A group of one-year-olds and parents celebrated the occasion

Despite opening five years late and �13m over budget the �35.6m tower has been hailed a success in terms of visitor numbers.

Since opening, the 170m (557ft) tower has been visited by more than 600,000 people, exceeding the 460,000 predicted by owners Portsmouth City Council.

But it has not all been plain sailing as an external lift has yet to open.

The lift jammed 100ft (30m) up, trapping the city council's project manager, David Greenhalgh, at the opening ceremony.

A spokesman for the tower, England's tallest publicly accessible building outside London, claimed that 50% of visitors came to Portsmouth specifically to visit the tower.

It has been largely welcomed by city residents, who funded the project through increased council tax rates, although some doubters do remain.

Councillor Gerald Vernon-Jackson, the leader of Portsmouth City Council, said it was time to move on from the debate over the tower's existence.

"It's a wonderful building and a great symbol putting Portsmouth firmly on the map," he said.

"It's one of the attractions that makes the city so great."

On Wednesday a group of children, who are all one-year-old, and their parents held a joint birthday celebration on one of the tower's viewing platforms.