I am 40 weeks pregnant and have had a continuous dull ache in my lower back for last 3 days. Is this a sign of labour approaching?

It could be, as the nerves that supply your cervix with sensation are the same as those that supply your lower back. So if there is pressure on your cervix and it is starting to dilate (open) even without contractions, this could be felt in your lower back. The alternative exlanation is lower back pain, which is common as baby's head begins to engage.