It largely depends on the sentence in which you're using your particular example, but there is a case where using 'fewer than' instead of 'less than' is correct.

• 'Less' means not as much
• 'Fewer' means 'not as many'

For example, if I'm holding three apples I have 'fewer than 4 apples'.

If I'm holding half a kilogram of sugar, I have 'less than a kilogram of sugar'.

Here's a BBC article highlighting this often-made mistake: http://news.bbc.co.uk/1/hi/magazine/7591905.stm

You would say less than or the lesser of. Not lesser than.

However, it largely depends on the sentence in which you're using your particular example, as it may be that using 'fewer than' instead of 'less than' is correct.

• 'Less' means not as much
• 'Fewer' means 'not as many'

For example, if I'm holding three apples I have 'fewer than 4 apples'.

If I'm holding half a kilogram of sugar, I have 'less than a kilogram of sugar'.

Here's a BBC article highlighting this often-made mistake: http://news.bbc.co.uk/1/hi/magazine/7591905.stm