verbs - "Increase 10%" or "increase by 10%"

I was taught at the university that the correct usage of this verb is increase by, for example, like in the sentences stated below:

The company's income increased by 10%.

By 2015 the population of this city will have increased by 10%.

Nowadays I see the verb increase used without any prepositions and I wonder if anyone knows if there is any difference between these two options and whether both of them are considered to be gramatically correct.