Sex and the City 2 beaten by Shrek at US box office

Shrek Forever After is voiced by actors including Mike Myers

Shrek Forever After beat competition from the new Sex and the City film and Disney's Prince of Persia to stay top in North America, early estimates show.

The fourth instalment in the animated franchise took $55.7m (£38.4m).

Prince of Persia opened at number two, with $37.8m (£26.1m), beating Sex and the City 2, which took $37.1m (£25.6m).

Paul Dergarabedian, of box office tracker Hollywood.com, said family films had been most popular because it had been a holiday weekend in the US.

The Memorial Day weekend is traditionally one of the most successful of the year for ticket sales.

But ticket sales this year dropped to $182.2m (£126m) from $213.8m (£147.8m) on the same weekend last year.

"The audience is not absolutely thrilled with the movies in the marketplace and they're voting with their absence now," Mr Dergarabedian said.

Warner Bros said many women movie-goers would watch the much-anticipated but critically-panned sequel to 2008's Sex and the City during the week.

"We will do better midweek," domestic distribution head Dan Fellman said.

"It's just that this was a little more difficult weekend."

Iron Man 2 dropped two places to number four, taking $20.6m (£14.2m).

The top five is rounded out by Robin Hood, which took $13.6m (£9.4m).