The big "No-No" - reneging on an offer

Ok, here it goes. I've always wanted to do equity research and applied for internships at a heap of places this past recruiting season. As the interviews were scarce I also started applying at other places, among others Bloomberg and got an offer from them for a London-based internship in Analytics. I signed the contract but was then invited to an interview with a boutique equity research firm on the continent (France) and they gave me a verbal offer. So, the question is, is it worth it to renege on the Bloomberg offer to go for equity research at a boutique firm, if my long-term goal is public buy-side (HF or AM)? Obviously Bloomberg is a much longer shot to that destination, but is it worth taking the shitty step of reneging on a signed offer? Also, what are possible ramifications? Thanks!