I'm due to start my PhD in 3 weeks time, but received an email from my supervisor a couple of days ago to tell me that they have just been offered a job elsewhere, and my project is now off the table. The university are still going to honour my offer of a studentship, but I don't know for what project or who my supervisor will be.

My 2nd supervisor (who I haven't met) is apparently 'very keen to work with me', but they are a post doc, and reading between the lines, I suspect that they will be my de facto supervisor, while somebody more senior will be a nominal 1st supervisor. I may be reading too much into this though.

Another alternative suggested is that I join a completely different group, but none was actually suggested. For the moment, it's being left for discussion once I arrive.

Has anybody else found themselves in this position before, or have any advice on how I should handle this?

No, I haven't found myself in this position, but I think it is clear that you need to be contacting the University / Department with some urgency to get to the bottom of it.

I wouldn't worry about it. Even projects with a supervisor and a supposed trajectory can end up being wildly different to what was proposed. Supervisors tend to think about things at the last minute, so it's not a problem if you get an actual project a few weeks into the PhD.

The postdoc will probably have more time and be more keen to supervisor you than a typical prof/reader/senior lecturer would have/be, so I don't think this matters.

Ask them what potential projects are on offer, or suggest your own ideas if you have them, and see what they say.

Thanks for the input. I guess it will be better to wait and have a chance to get to know the people concerned rather than pushing for an early decision. It's just rather unsettling at this point, and I feel that I don't know enough about the whole process to understand the pros and cons of seniority in a supervisor.

Well, it's a bad condition. I understand but don't worry at all! First, you should know that a good PhD supervisor is a key to future employment. So, a postdoc can be very helpful in guiding you during the projects but he/she will not be able to help you find a good job or position. Your next move should be contacting your first supervisor and ask him if he suggests another faculty member of the department. Clarify for him the subjects you are interested in. If he didn't write you responsibly in finding you a good supervisor, then you should act almost independently. Go through papers of the faculty members, check their group members and find some candidates. Then, with the help of your department contact them. In such situations, one of the faculty members will for sure accept you. My experience says that the best thing will happen to you and after graduation you will appreciate this awful condition.

Thank you for the advice. I'm currently investigating the faculty members' papers and bios to get an idea of who I would be happy to work with, before taking any further steps.