I am a PhD student in the UK due to start a three-month internship in a startup within the next two weeks. When I was made an offer (a couple of months ago), the company pushed very hard to offer me a permanent position. I insisted on the three-months internship because this is my first job post-PhD.
After signing the contract (with the mutual understanding that if all goes well I will stay for a permanent position after the internship), I got an opportunity to spend three months in a research group. This is a professional opportunity that I absolutely do not want to pass on, so I accepted, and am due to start right after the three months internship.
When should I tell the startup bosses that I am definitely leaving for three months after the internship? Depending on how it goes I wouldn't mind then coming back to the startup and taking on a permanent role, so I would like to roll this out in the best way possible.
UPDATE I ended up telling them during my second week, mentioning it as an opportunity I hadn't yet accepted but did not want to refuse, and asking them how open they would be to a delayed start to my permanent contract. They served me my notice immediately, and seem intent in giving me no meaningful work for the next month. In hindsight, I should have told them much later, as that would have given me two more paid months.