Britain's prime minister David Cameron is paying a farewell visit to Queen Elizabeth II as the country begins formal campaigning for an election seen as the most unpredictable in decades.

The audience comes as Britain's Parliament was dissolved ahead of the May 7 vote.

Voters failed to produce a majority for the Conservative Party or the Labour Party in the House of Commons in 2010, and the electoral landscape is fractured again. Minor parties — such as the Scottish and Welsh nationalists, Greens and anti-Europeans — could hold the balance of power.

Cameron and opposition leader Ed Miliband are focusing on the economy Monday. Cameron is warning of economic chaos under Labour while Miliband says the Tory position on a European Union referendum offers "clear and present danger" to business.