Question

According to Newton’s first law of motion, what will an object in motion do when no external force acts on it?

Answer 1

An object will continue in constant, uniform motion (in a straight
line at constant speed) until acted on by a net external force.

The speed may just as well be zero.  That covers the separate case
that people always want to add, of "an object at rest".

Answer 2

By definition, we have to:

Newton's first law states that any object will remain in a state of rest or with a uniform rectilinear motion unless an external force acts on it.

Therefore, according to the first law of Newton, if the object is already in motion and has no force acting on it then, it will remain with a uniform rectilinear motion.

Answer:

The object will remain with a uniform rectilinear movement when the external force does not act on it.