Britain's Prime Minister David Cameron and wife Samantha speak to members of the public as they walk around the Wells May Day Fate in Bath, Britain May 4, 2015. Britain will go to the polls in a national election on May 7. [Photo/Agencies]

Ahead of Britain's parliamentary election on Thursday, Prime Minister David Cameron's Conservatives and the opposition Labour Party are tied at 33 percent, according to a YouGov opinion poll for the Sun newspaper published on Monday.

The poll showed Conservatives down 1 percentage point from Sunday versus Labour who remained unchanged, polling company YouGov said.

The election is expected to be the tightest in decades.

The two main parties have been neck and neck in most opinion polls since the start of the year, with neither establishing a sustained lead exceeding the three-point margin of error.

Opinion polls have consistently shown that neither of the two main parties is likely to win an overall majority in the 650-seat Parliament.