By Jodie Ginsberg
LONDON (Reuters) - Britain's opposition Conservative party is gaining ground in pivotal parliamentary seats and could secure a narrow but outright win in Thursday's general election, a poll published on Monday showed.
The final Reuters/Ipsos MORI marginals poll, which surveys voters living in the kind of constituencies held by the ruling Labour party that the Conservatives need to win for a majority, shows support for the parties in these seats is neck and neck.
That represents a 7 percent switch in support to the Conservatives from Labour compared to the 2005 election and could be just enough to put them into power in the 650-seat parliament.
"These findings show the Conservatives on the verge of winning enough seats to secure the narrowest of majorities," said Roger Mortimore, Head of Political and Electoral Research at Ipsos MORI.
However, he said that with a third of voters still prepared to change their minds, a more comfortable Conservative win or the first inconclusive election since 1974 were possible.
The poll indicated the Conservative majority could be as small as two seats, meaning the center-right party could on occasions need the support of minor parties such as Northern Ireland's pro-British unionists to insulate them against losses in tight votes.
Labour, in power since 1997, will send a letter to 850,000 voters on Wednesday across more than 100 Labour-held seats urging those considering voting for the centrist Liberal Democrats not to split the anti-Conservative vote.
Conservative leader David Cameron had claimed to have the momentum after a strong performance in last week's leaders' television debate.
However, surveys published earlier on Monday indicated his party's lead had been pegged back to 5 percentage points -- as little as half their weekend advantage -- and suggested either the Conservatives or Prime Minister Gordon Brown's Labour Party could still capture most seats in the 650-member parliament.
Cameron, speaking in the northern English seaside town of Blackpool on a public holiday, acknowledged that millions of Britons had not decided how to vote. He pledged to campaign all night this week to try to win over waverers.
"This election is not yet won, but if we get out there, we can -- and I use the word can, I always use the word can, never the word will -- win it."
The quirks of Britain's electoral system, where seats are allocated purely by constituency results, and not in proportion to the overall share of the vote, mean that Labour could come third in the popular vote but still remain the largest bloc.
The race has been blown wide open by a strong showing from the Liberal Democrats, traditionally Britain's third party, whose own telegenic leader, Nick Clegg, has challenged Cameron's claim to be the candidate of change.
Clegg, whose centrist party has taken a tough line on bank bonuses and wants to reform the electoral system, told potential supporters not to lose their nerve.
"We have an opportunity of a lifetime, a once-in-a-generation chance, to change Britain for good," he said during a campaign stop in south London.
Markets are concerned that an inconclusive election could complicate efforts to tackle a budget deficit running at in excess of 11 percent of GDP.
Influential U.S. investor Jim Rogers, who has long been negative about the outlook for the pound, said politicians from all the major parties had failed to spell out the depths of the economic problems Britain faces.
"I don't see a solution on the horizon, I don't see any politician in the UK acknowledging the extent of the problems or willing to do anything about it," he told BBC Radio 4.
(Additional reporting by Tim Castle, Keith Weir and Avril Ormsby)
(Editing by Ralph Boulton)