The Member of Parliament for Kensington and Chelsea, the Rt. Hon. Alan Clark, (Conservative) died of a brain tumour on September 5, 1999.
This was the first truly safe Conservative seat to have a byelection in the Parliament as well as the first in London. There was immediate speculation that Michael Portillo, the most high-profile casualty of the 1997 general election, would use it to return to frontline politics. Portillo immediately confirmed his interest in the seat, but was then confronted with the publication of an interview he had given previously that summer in which he had confirmed that while at Peterhouse, Cambridge he had had homosexual affairs.
Portillo was selected as Conservative candidate but faced demonstrations organised by gay rights group OutRage! and its principal campaigner Peter Tatchell who protested against his vote for an unequal age of consent for gay and straight sex, and support for the ban on homosexuality in the UK armed forces while Secretary of State for Defence. Tatchell continued to try to confront Portillo throughout the election, not assuaged by Portillo saying that he had changed his mind on the age of consent.
The Labour Party selected Robert Atkinson, who had fought the 1997 election and was a local councillor. The Liberal Democrats also renominated their general election candidate, Robert Woodthorpe Browne. Because of the prominence of the byelection in central London and the big political name, there were a wide variety of fringe and minor party candidates. Polling day was set for November 25. Michael Portillo returned safely to Parliament.
Results
The turnout was 29.7%.
| Candidate | Party | Votes | Share | Change |
| Michael Portillo | Conservative | 11,004 | 56.4% | +2.8% |
| Robert Atkinson | Labour | 4,298 | 22.0% | -5.9% |
| Robert Woodthorpe Browne | Liberal Democrat | 1,831 | 9.4% | -5.9% |
| John Stevens | Pro-Euro Conservative Party | 740 | 3.8% | |
| Damian Hockney | UK Independence Party | 450 | 2.3% | +0.9% |
| Hugo Charlton | Green Party | 446 | 2.3% | |
| The Earl of Burford | Independent | 182 | 0.9% | |
| Colin Paisley | Legalise Cannabis Alliance | 141 | 0.7% | |
| Dr. Michael Irwin | Independent | 97 | 0.5% | |
| Paul Oliver | UK Pensioners Party | 75 | 0.4% | -0.1% |
| Stephen Scott-Fawcett | Independent Labour | 57 | 0.3% | |
| Louise Hodges | Independent | 48 | 0.3% | |
| Gerard 'Ged' Valente | Natural Law Party | 35 | 0.2% | -0.1% |
| Lisa Lovebucket | Pro-Life Alliance | 26 | 0.1% | |
| John Davies | Independent | 24 | 0.1% | |
| Peter May | Independent | 24 | 0.1% | |
| Howling Laud Hope | Monster Raving Loony | 20 | 0.1% | |
| Tonysamuelsondotcom | Independent | 15 | 0.1% |
General Election result, 1997
| Candidate | Party | Votes | Share |
| Alan Clark | (C) | 19,887 | 53.6% |
| Robert Atkinson | (Lab) | 10,368 | 27.9% |
| Robert Woodthorpe Browne | (L Dem) | 5,668 | 15.3% |
| Angela Ellis-Jones | (UKIP) | 540 | 1.5% |
| Edward Bear | (Ind) | 218 | 0.6% |
| Paul Oliver | (UKPP) | 176 | 0.5% |
| Susan Hamza | (NLP) | 122 | 0.3% |
| Paul Sullivan | (Rainbow) | 65 | 0.2% |
| Pete Parliament | (Ind) | 44 | 0.1% |