John Howell, a former partner in City accountants Ernst & Young, was chosen from a shortlist of three by Henley Conservative Association.

Mr Johnson is standing down shortly to concentrate on his new job as Mayor of London, after beating predecessor Ken Livingstone earlier this month.

The Henley MP is yet to formally quit, although the Conservatives are expected to trigger a by-election for late June or early July.

The contest will be another test for Prime Minister Gordon Brown after Labour was humiliatingly defeated in the Crewe and Nantwich by-election this month.

Tory leader David Cameron will be seeking to build on his party's success there by defending his party's 12,793 majority in Henley. Nick Clegg's success so far at the helm of the Liberal Democrats, who were the closest competitors for the seat in the 2005 general election, will also be under the spotlight.

A cabinet member on Oxfordshire County Council, Mr Howell was awarded an OBE in 2000.

The Tories said he would be a champion for the constituency.

"A successful businessman, he is leading the way in ensuring the county council is run more efficiently and in opposing the bulldozing of the green belt," a spokesman said.

Mr Howell said: "For now I just want to say that it is a huge privilege to have been selected as the Conservative candidate for my home constituency. I plan to spend the next few weeks talking to as many local people as possible about the issues that affect us all in South Oxfordshire."