[R] Post-hoc test on ANCOVA

[Mark Na]

Dear R-helpers,

I have an ANCOVA with a significant effect of the factor, which has
three levels. I wish to determine which of the levels are different
from each other but, because my model was fitted with lm(), I cannot
use TukeyHSD.

For some reason, I get different results (no significant effect of the
factor) when I fit the model using aov() so, for the moment, I am
using lm().

Could anyone point me to a test and associated R function that will
work on a fitted lm() or glm()?

Many thanks,

Mark Na