British general election campaign formally under way

British prime minister David Cameron has visited the Queen at Buckingham Palace to ask her to dissolve parliament, a move that formally begins the election campaign.

Mr Cameron left Downing St tonight to visit the monarch.

There are now 38 days of campaigning ahead of the May 7 poll, tipped to be the tightest in a generation.

Mr Cameron has already been on the offensive against Labour rival Ed Miliband, warning the opposition would introduce a £3000 (A$5793) tax on families should they win office.

"In thirty eight days’ time you face a stark choice,” Mr Cameron said outside Downing St.

“The next Prime Minister walking through that door will be me or Ed Milliband.”

The prime minister has claimed taxes will not rise under his Tory government should they be re-elected.

Mr Miliband, meanwhile, has warned the proposed 2017 referendum on Britain’s membership of the EU could lead to an "extraordinary loss of British influence".

Today’s development follows weeks of unofficial campaigning on all side.