British Prime Minister Boris Johnson will formally announce a December 12 election on Wednesday, urging voters to back him and promising to "get Brexit done in the next few weeks."

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The early election was agreed by parliament last week, ending two years of deep disagreement over Brexit that has sapped investors' faith in the stability of the world's fifth largest economy and damaged Britain's international standing.

The outcome of the vote is hard to predict, with the immediate question of Britain's exit from the European Union scrambling voters' traditional loyalties and giving smaller rivals a chance to challenge the two biggest parties, Johnson's Conservatives and the left-of-centre Labour Party.

After dissolving parliament and paying a formal visit to Queen Elizabeth on Wednesday, Johnson will return to his Downing Street residence to announce the election. He will launch his party's campaign at a rally later in the day.

"I don't want an election. No prime minister wants an early election, especially not in December," he will say, according to advance extracts of his speech.

"But as things stand we simply have no choice – because it is only by getting Brexit done in the next few weeks that we can focus on all the priorities of the British people."

Polls show Johnson's Conservatives are well ahead of Labour, but polling analysts caution that sentiment is unusually volatile.

Johnson, 55, is hoping to win a big enough majority in parliament to get the Brexit deal he agreed with Brussels last month ratified and lead Britain out of the EU at the end of December or in January.

Britain had been due to leave in March, but has had to extend that deadline three times after parliament rejected a deal negotiated by Johnson's predecessor Theresa May, and then forced Johnson himself to ask for more time.

His campaign launch will paint Labour Party leader Jeremy Corbyn as someone who wants to stall the Brexit process by holding another referendum, holding the country back from focusing on domestic issues.

Corbyn said on Tuesday he wanted to negotiate a new Brexit deal and then let the public decide between leaving on his terms, or not leaving at all. He has accused Johnson of negotiating a bad deal that would hurt the economy and erode workers' rights. 

Attempting to capitalize on voters' Brexit fatigue, more than three years after Britain voted 52 percent to 48 percent in favor of leaving the bloc, Johnson will say: 

"It's time to change the dismal pattern of the last three years and to get out of our rut. It's time to end this debilitating delay."

(With input from Reuters)