Wanted: volunteers to become a public work of art for an hour at a time in Trafalgar Square. Clothing optional.

Antony Gormley's idea to occupy the vacant Fourth Plinth in the square with a succession of members of the public was greeted with enthusiasm yesterday morning by Boris Johnson, the Mayor of London, but by the afternoon his office was sounding more concerned.

Gormley, who is best known for creating The Angel of the North, announced at a press conference that volunteers should come from all walks of life, including nudists. Unveiling the project, titled One and Other, Gormley listed the type of people he would like to see on show: “We need the bodybuilder but we also need the paraplegic, the naturist, the Shakespearean actor ...”

Scotland Yard, when contacted by The Times, suggested that if the 57-year-old artist's vision was fulfilled there was a likelihood that exhibits would be arrested. “Should that arise and there were complaints from members of the public during the art exhibition - for want of a better word - it would be assessed at the time and appropriate action taken,” a spokeswoman said.

Guto Harri, the Mayor's spokesman, conceded that it would be impossible to engineer the type of people who take part in the project, which will run 24 hours a day for 100 days, because volunteers would be selected in a lottery. “Would you want a naked man or woman in the middle of Trafalgar Square? It's unlikely. But we'll cross that bridge when we come to it.”

There have been nude statues in London since 1822, when Sir Richard Westmacott's statue of Achilles was unveiled to public consternation in Hyde Park, but this would be the first instance of a nude, live model being sanctioned in the capital. Gormley, who set up 31 statues of his own naked body in London last year as part of an exhibition for the Hayward Gallery, said that volunteers would not be exempt from public decency laws despite their status as works of art.

“I think they will be subject to the law even when they are on the plinth. The law will still be the law on the plinth.” The exhibits will be supervised by six curators who will be on hand at all hours.

The artist, who will receive a £30,000 fee as part of the £170,000 of public money awarded for the commission, said that the method for placing each of the 2,400 participants on the plinth had not been decided.

“We presented a hydraulic stair but I'm rather favouring a crane so there is a moment of theatre every hour. There would be a rather nice moment, like going into the crease to bat, where the person leaving and the person arriving will meet each other.”

The mechanism for selecting volunteers has not been finalised, but the public will be invited to take part in a national competition. Organisers suggested that it would be possible to apply using the fourth plinth website.

The project is expected to begin next summer after the current commission, Thomas Schutte's Model for a Hotel, is removed. Yinka Shonibare, nominated for the Turner Prize in 2004, also received a commission yesterday for the plinth.

His proposal, Nelson's Ship in a Bottle, is a scale replica of HMS Victory with sails made from fabric printed in a style associated with Africa and placed in a giant bottle. He described it as celebrating “London's immense ethnic wealth”.

The selection, from a shortlist including Jeremy Deller, Tracey Emin, Anish Kapoor and Bob and Roberta Smith, was made by a panel chaired by Ekow Eshun, artistic director of the Institute of Contemporary Art in London. London is not known for its tolerance of nudity in public art. Sir Richard's statue of Achilles was dismissed as nonsense by The Times and its private parts were later shielded with a fig leaf.

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