British Prime Minister Theresa May's lead over the opposition Labour Party has narrowed sharply, according to five opinion polls published since the Manchester attack, suggesting she might not win the landslide predicted just a month ago.

Four opinion polls published on Saturday showed that May's lead had contracted by a range of 2 to 6 percentage points, indicating the June 8 election could be much tighter than initially thought when she called the snap vote.

"Theresa May is certainly the overwhelming favorite to win but crucially we are in the territory now where how well she is going to win is uncertain," John Curtice, professor of politics at the University of Strathclyde, told Reuters.