[dodgyb]

Be prepared for the questions that you will be asked when the alarm is raised.

Q1. Why late? Work is 'outside of comfort zone', thus accurate estimates are impossible.

Q2. When will it be ready? Scope, timescale and quality are interdependent. Ask 'the business' which two are most important to them. The remaining item will need to be revised. My recommendation would be to curtail scope and deliver the rest in phase 2. If they say timescale can be extended the revise estimates based on current velocity, i.e. if you are 30% behind where you expected, add 30% to all outstanding work items. If quality is cut, make sure they are aware of the additional support and maintenance costs that will be incurred.

Either way, do as rinka_singh says and get the PO/PM to establish a risk log accessible to all. I would add that each risk should be assigned to someone who is responsible for tracking it. Review the log each week.

Raise the alarm now because there will be dependencies, and you are probably not aware of all of them.

I would not be worried about how the alarm is percieved, if your intentions are to steer 'the business' away from a cliff it will be recognized.