Milton Gordon laughed affectionately at the name. “We just wish there were two Borises!” he said. “We are sorry to see him go.”

His reaction, particularly the laughter, seemed almost universal on the sunny streets of Mr Johnson's Henley constituency. The new Mayor of London may be doing his best to cultivate an image of statesmanlike seriousness, but the residents of south Oxfordshire apparently still regard him as a cross between friendly MP and comic uncle.

In affluent Henley-on-Thames itself, people were more concerned with preparations for this summer's regatta than with the need to replace him.

He will stand down, and there will be a by-election, but the local Conservative Party says that it will not have a new candidate in place until the end of June. Its provisional date for a final selection meeting, June 27, would have matters wrapped up nicely just before regatta week. The poll itself may yet be several months away.Asked whether it will be a problem that Mr Johnson will have little time for constituency concerns, John Walsh, the Conservative agent in Henley, seemed remarkably unconcerned: “He'll have a bit less time yes, but primarily he's giving up a lot of other activities for the moment. We have always had MPs who are busy doing other things. Michael Heseltine [Mr Johnson's predecessor in Henley] was busy being Deputy Prime Minister.”

“I'm sure Boris will keep in touch,” said Mr Gordon, a financial adviser.

Josh Wood, who has just turned 18, was in no rush to exercise his democratic right for the first time. “Henley's a cool place to live. It's an easy life,” he said, suggesting that he might vote Conservative because of his parents' concerns about inheritance tax.

Twenty miles across the constituency, on a business park in Thame, the Liberal Democrats were displaying a somewhat greater sense of urgency. Despite not knowing when the vote will be, the party's two-storey by-election campaign headquarters opened for business on Friday. “We started getting ready last Friday [May 1], as soon as it became clear that Boris had won,” said Chris Leaman, a campaigns officer normally based in London, who is already sleeping on an airbed at the local party agent's house.

The Liberal Democrats know that interest in the by-election will pick up once the date is named. Neil Fawcett, the deputy director of campaigns, believes that the constituency is not as true blue as some might think and a long build-up to a by-election will benefit his party. “There is a vote that Mr Johnson got that isn't hardcore Conservative. I can't see why they would let us build up momentum,” he said.

The Liberal Democrats have to overturn Mr Johnson's majority of 12,793 if they are to take Henley, and they are determined that David Cameron's strategists will not catch them unawares by suddenly calling the poll.

The Lib Dems have already printed their first campaign leaflet,an attack on the Tories' environmental record, and while it has pictures of Mr Cameron, Nick Clegg and even a polar bear, there is not a word about the sitting MP. Any attacks on Boris would put the Lib Dems on a “hiding to nothing”, said Mr Fawcett.

On this they seem to be right. Even in Berinsfield, a village that is as close as the Henley constituency gets to potential Labour territory, Mr Johnson has won popularity for protecting facilities at the local primary school. Kerry Walls, 32, is usually a Labour voter but, pointing at her friend who is a single mother, says that she has gone off the party recently because they “give you money with one hand and take with the other”.

Back in Henley-on-Thames, octogenarians Rachel York and Murag Murr were taking the prospect of a delayed poll in their stride. “There are other people to keep an eye on things,” Mrs Murr said, while Mrs York was keen to defend the “lovely” Mr Johnson. “The poor man can't tear himself in half,” she said.

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