Over 44 million people who registered to vote are expected to vote for nearly 4,150 candidates in 649 constituencies from 7am (GMT) until 10pm.
Britons today voted in what is billed as the most tightly-contested general election since the World War II which could lead to a hung Parliament, though latest opinion polls gave a clear edge to opposition Tories over the ruling Labour as well as Liberal Democrats.
Polling stations opened at 7 am local time and the voters have until 10 pm to exercise their franchise, as police commenced a probe into alleged fraudulent postal voting and false registration of voters' addresses.
Over 44 million people were registered to vote for nearly 4,150 candidates in 649 constituencies. Polling in one constituency - Thirsk and Malton - has been delayed until May 27 because of the death of one of the candidates during the campaign.
First results are expected at 11 pm local time. The vast majority of constituencies will count votes and declare results overnight.
A string of eve-of-election opinion polls gave David Cameron's Conservatives a clear lead over Labour and the Liberal Democrats.
But they still suggest the Tories may not reach the level of support they need to claim an overall majority in the 650-member House of Commons. Cameron is likely to form a coalition government with MPs from Northern Ireland.
The result is the most uncertain in any election since 1992 and there is the first real prospect of a hung Parliament in almost 30 years.
According to the last polls of the campaign, Cameron is within touching distance of a Commons majority with the Tories set to make massive gains from Labour.
The surveys gave the Conservatives a lead of up to nine percentage points and provided fresh evidence that the Liberal Democrats poll bounce is fading.
Pollsters expect that with bigger swings in key marginal seats, the Tories will win between 300 and 310 seats - close to the 326 they need for an absolute Commons majority.
According to the latest Harris/Daily Mail, Tories may get 35%, Labour 29 and Lib Dems 27, while YouGov/Sun poll claimed 35% for Tories, 28 for Labour and Lib Dems each.
A Populus/Times poll indicated 37% for Tories, 28 for Labour and 27 for Lib Dems. ICM/Guardian poll showed 36% for Tories, 28 per cent for Labour and 26% for Lib Dems.
Exit polls will be broadcast on television immediately after voting ends at 10 pm.
Meanwhile, the Metropolitan Police here said it had received a number of allegations concerning false registration of voters' addresses in Tower Hamlets.
Its four ongoing investigations in the borough were being combined into one probe, along side a second investigation into election irregularities in Ealing.
A total of 23 allegations of election irregularities across the capital were currently being assessed.
The allegations relate to 12 boroughs -- Tower Hamlets, Lambeth, Barking and Dagenham, Westminster, Enfield, Hounslow, Haringey, Bexleyheath, Camden, Redbridge, Hillingdon and Bromley.
Greater Manchester Police said it had received reports of electoral fraud in the Rochdale, North Manchester, Oldham and Bolton areas, relating to breaches of the electoral Code of Practice.
The force said officers were working with the Electoral Commission and local election officers to establish if any
crimes had been committed.
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