The latest aid for politicians on the campaign trial requires no hard drives, is easily portable and adds instant gravitas to serious moments.

A pair of spectacles is emerging as the must-have tool for top politicians who want to appear focused on the issues that matter.

Tony Blair began the trend just weeks before he called the election when he "came out" at a speech to regional business leaders and produced a pair of fashionable glasses.

He explained that Cherie had persuaded him to use glasses because he could no longer define millions and billions without them.

"The prime minister was prescribed with glasses in 1998 but has only begun wearing them in public recently. They are British made," a Downing Street spokesman confirmed on Thursday.

Wearing glasses is only part of the image boost. Holding them at key moments in press conferences is an added bonus. Image advisers have hit on the idea that a pair of glasses in your hand makes better TV pictures and conveys a "serious" message.

During the launch of his party's election manifesto, shadow chancellor Michael Portillo could be seen looking intently with a furrowed brow whilst chewing the arm of his glasses.

A spokesman for the Conservatives denied there was a new strategy. "There's nothing new in this - the shadow chancellor has been wearing glasses for years."

Not to be outdone, a spokesman for the Liberal Democrats confirmed that several members of the party's front bench team sport specs.

"Charles Kennedy doesn't wear glasses but Don Foster, Alan Beith and Menzies Campbell all do," he said.

Ian Wright, of the Institute of Public Relations, said this election will see politicians struggling to project a range of images.

"Using glasses is a visual cue. It's also a way for politicians to make a virtue of advancing age - both Portillo and Blair are in the same age bracket," he said.

Wright added that the rise of the "dress-down" culture will add to the presentational headache.

"The haircut used to be the issue. This time there's going to be a lot of issues like jackets on or off. Tony Blair is slightly buttoned up so he likes to look casual. The pressed jeans may make an appearance for the weekend campaigning," he said.

"The rise of dress down over the last three years presents an exquisite dilemma. Politicians want to be seen as business-like but have to be casual as well. This is the first baby-boomers election and it will be interesting to see which camp the leaders side with."