If it was a condition of your offer that you have life assurance in place, yes you will be in breach of the mortgage conditions if you cancel your cover.
What will happen? The life assurance company (provided there is an assignment on the policy which there more than likely is or should be) will write to you and copy the lender on your correspondence. The lender will then write to you asking if you have other cover in place. What other action they take depends on the lenders policies.
BTW do you have dependants? If you do, then you really need to think of the consequences of cancelling your cover when you have a mortgage.