British Prime Minister Gordon Brown has arrived at Buckingham Palace where he is expected to ask the Queen to dissolve Parliament and call a general election for May 6.
Unofficial campaigning started weeks ago, but the announcement will dramatically up the stakes in a battle which sees David Cameron's Conservatives hoping to take power for the first time in 13 years.
In an election likely to be dominated by the economy, Brown, 59, is contrasting his role in steering Britain to economic recovery after the global financial crisis with what he says is 43-year-old Cameron's inexperience.
"The people of this country have fought too hard to get Britain on the road to recovery to allow anybody to take us back on the road to recession," he is set say after announcing the election date.
Meanwhile Cameron, who has extensively modernised the once pro-market party of Margaret Thatcher since taking over as leader in 2005, will say he is "fighting this election for the Great Ignored -- young, old, rich, poor, black, white, gay, straight".
Cameron's centre-right Conservatives had established a long-term double-digit lead over Brown's centre-left Labour Party before January's announcement that Britain had emerged from its worst recession since World War II.
That then melted away to single figures but has begun to widen out again in recent days.
A survey for the Daily Express newspaper yesterdy gave the Tories a commanding 10-point lead, which would probably give them a majority in the Commons.
But in a sign of the variations in opinion polls, a survey for the Guardian newspaper the same day showed Labour closing the gap, just four points behind Cameron's party.
Whoever wins faces having to tackle a crippling budget deficit of at least 167 billion pounds (254 billion dollars, 188 billion euros) and a fragile economy which some experts say could still dip back into recession.
Brown is fighting his first general election as prime minister, having only taken over from Tony Blair in June 2007.
The Conservatives need a huge swing of 6.9 percent to secure victory -- equivalent to the landslide which swept Labour led by Tony Blair to power in 1997.
Labour currently has 345 seats in the House of Commons, a working majority of 56, compared to the Conservatives' 193. It secured three clear victories in general elections in 1997, 2001 and 2005.
If, as polls suggest is possible, no one party achieves an overall majority, the third party centre-left Liberal Democrats could play a key role in a minority or coalition government.
Their leader Nick Clegg told reporters: "Today is the beginning of the end for Gordon Brown".
He added: "Now is the time for all those people who want real change and real fairness in Britain to choose something different and turn to the Liberal Democrats."
A highlight of the general election campaign is likely to be the first-ever television debates between the men vying to be prime minister.
Brown, Cameron and Clegg will go head-to-head in three live 90-minute programmes.
After Brown announces the election date, parliament will not be dissolved immediately -- that is expected to happen on Monday.
Instead, there will be a period of several days of "wash-up" -- a time when loose ends of legislation are tied up and parties barter to get certain pieces of legislation passed.