I don’t know and haven’t had a critical illness policy but I would guess so. Smoking is reckoned as an environmental (that is, non-genetic) factor that increases the probability of developing MS (there was a post in March on “Smoking increases your chance of getting MS” on the Barts multiple sclerosis research blog [the head of the team is also a clinical neurologist]). If the medics and scientists think there is a link, I think the insurance industry will have factored it in. If you could manage to stop now you could answer truthfully in the future that you don’t smoke. Also stopping now is likely to be of benefit with respect to MS in the future.
Giving up is difficult though.