Members of the Rail, Maritime and Transport union voted by 2,614 to 643 for industrial action in a dispute over pay and conditions. The union yesterday said June 10 would be the first of 24-hour stoppages by station staff.
Mr Livingstone condemned the stoppage, saying a strike at that date was unnecessary and unacceptable. "The RMT should return to the negotiating table with London Underground."
A source said the mayor was furious at the development: "He takes the view that he's gone out of his way to be fair to trade unions and he doesn't need them jeopardising his hopes of re-election."
The RMT's general secretary, Bob Crow, is on the board of Mr Livingstone's Transport for London authority.
Insiders claimed Mr Crow had argued for a delay in industrial action. But he was said to have been narrowly overruled by the union's national executive. Mr Crow said the timing of the election was "not a big deal because the majority of Londoners aren't going to vote anyway".
The RMT has rejected a 3% pay offer. It wants a rise in the minimum salary for station staff from £19,000 to £22,000.
There was still time to avert the walkout with talks, Mr Crow added.
Steve Norris, the Conservative mayoral challenger, said: "Londoners who are frustrated and unable to travel that day will know where to put their crosses. This is a huge boost for our chances on June 10."
The RMT has threatened to co-ordinate future stoppages with strikes across the national rail network, as part of a separate dispute over pensions and travel benefits with Network Rail.
Mr Livingstone has historically been close to Britain's trade union movement. But some leftwingers have expressed disappointment with his re-entry to the Labour party. In the mayoral election, he faces a leftwing challenge from George Galloway's Respect coalition, which is campaigning on the basis of opposition to the war in Iraq.
