London ? The mayor unveiled plans Friday to bring back the beloved red double-decker Routemaster to London’s streets, but his opponents say the appeal to nostalgia throws pragmatism under the bus.

Three years after the buses were all but banished from the city, Mayor Boris Johnson hopes the new versions — whose open back allows passengers to hop on or off — will be up and running by the 2012 Olympics.

The last Routemasters were retired from regular service in 2005 in favor of safer and more modern models.

“I promised Londoners that I would hold a competition to design a new bus for London, based on the much-missed Routemaster, and today we can unveil the fantastic winning designs,” Johnson said.

More than 700 entries were received. Only two won.

One design by luxury auto maker Aston Martin and architecture firm Foster & Partners looks like a deluxe version of the old standby. Passengers sit on reconstituted leather upholstery while gazing through a glazed roof coated with solar panels to power heat and air conditioning. The rounded bus also boasts wood flooring and a saloon-like lower deck that the architect hopes will create “a tactile living room feel.”

A second design, from automotive designer Capoco Design Ltd., showed a more traditional-looking model with a front engine grille similar to the Routemaster style.

Neither entry estimated the costs associated with the designs.

The city expects to award a contract to develop and build the first new bus by the end of 2009, with the new vehicles on the street by 2011 in time for the 2012 Olympics, according to a Transport for London official. It wasn’t clear how quickly the new buses would be rolled out.

Cost is likely to be an issue.

Ken Livingstone, Johnson’s opponent in the mayoral race, estimated the plan would end up costing about $160 million a year, which he said would mean doubling bus fares. Johnson insisted the cost was much lower and could be met from the extra money collected through a crackdown on fare evasion.

Transport for London refused to estimate costs for the project, saying a final design hadn’t been chosen.

Johnson, who is frequently seen riding a bicycle around London, wants the new bus to be environmentally friendly.

Although a few vintage Routemasters still ply the tourist route between St. Paul’s Cathedral and the Tower of London, the bus once synonymous with London was gradually edged out by concerns over accessibility and cost.

Passengers loved the buses’ curved lines, the conductors and the open platforms at the back — ideal for a harried commuter racing to make a connection.

But the buses weren’t wheelchair-accessible, and the aging fleet proved expensive to maintain. Even the handy hop-on, hop-off platforms were a problem: Passengers racing to catch their early morning ride or sozzled after a night at the pub sometimes fell off the bus, with several reported injuries or deaths a year.

After the 1980s, the Routemasters were increasingly edged out by bigger, boxier buses, which kept the red color and the double decks but scrapped nearly everything else. More controversially, articulated single-decker vehicles nicknamed “bendy buses” were put on to some of the city’s busiest routes.

Detractors said the 60-foot-long buses weren’t adapted to London’s narrow streets, and they get into more accidents.

Johnson has called them “jackknifing, traffic-blocking, self-combusting, cyclist-crushing bendy buses” and vowed to get rid of them.