Record label bosses have told the four single lads in the band – Niall Horan, Harry Styles, Zayn Malik and Louis Tomlinson – to lay off the nookie to make sure they appeal to the American teen market.

Styles, 18, who is newly single after he split with The Xtra Factor host Caroline Flack, is reportedly furious at the new rules.

A source said: "He's come out of a relationship and wants to enjoy his freedom. But he's having to behave himself and keep the women at arm's length.

“They are all young lads and they have urges like anyone else their age.

“And the worst thing is the American girls are literally throwing themselves at them so there is so much temptation in their way."

Their debut album Up All Night went straight to No 1 in the US charts last week, making British/Irish boy band history.

Their three-month US tour starts in May, which will see the lads play 20 venues.

But they’ve been warned not to step out of line, because the American market won’t tolerate them being anything but squeaky clean.

The source added: "Niall and the boys are being kept on a really tight leash and it is tough for them. But they are so determined to make it work in the States they'll do whatever they are told.

"Quite simply a sex scandal over there would be the end of the band.”