Saving the planet isn’t always as straightforward as you’d think. Just ask McDonald’s British division, now reeling from the Great Drinking Straw Debacle.

In a bow to green sentiment (and a pending legal deadline), the chain last fall started banning plastic straws from its 1,300 UK outlets, replacing them with paper ones.

Problem #1: The new straws quickly got too soggy to use; 51,000 people signed a petition demanding the plastic ones back. “I’m all about saving the turtles,” tweeted one customer, but “I take 2 sips and my straw starts sticking together.”

So Mickey D’s made them thicker.

Oops: Our sister paper, The Sun, got its hands on an internal company memo revealing that the thicker straws . . . aren’t recyclable.

A McDonald’s spokesman had to admit to the press that straws by the millions “should be disposed of in general waste until further notice.”

Since the old plastic straws were 100% recyclable, the whole thing looks like a net loss to the environment.

This is hardly the only “green” advance that’s less than it seems: Even electric cars can be losers if, as in Germany, the power that charges their batteries comes mostly from coal.

Be careful what you crusade for.