Entering Economics Ph D from M Sc Behavioural Science I am an Economics undergraduate from India and I wish to at the end become a researcher in Economics after having a Ph. D. in Economics itself.
But given my interest in Behavioural Economics, I applied for and received an offer of admission for an M. Sc. in Behavioural Science (the institute is ranked in top 20 for both Economics and Behavioural Science globally by several organisations), so that I get a better view of the psychological side of things as well.
But I am worried if this temporary detour can negatively affect my prospects for getting admitted into a well ranked Ph. D. programme in Economics. In particular, I have heard that mostly candidates from extremely math heavy backgrounds are preferred for Economics Ph. D. courses and M. Sc. Behavioural Science may not be counted as a particularly math heavy programme, although the programme does include a fair amount of both Economics and Statistics.
Are my concerns valid? Will a master's in Behavioural Science seriously negatively affect my prospects? Should I go ahead with my plan or not?
I certainly don't see a MS in Behavioral Science as a negative, and I don't think any econ PhD program that does research in behavioral economics will either.  The reason you're interested in it is the same reason it applies well to that subfield.  That said:

You may make yourself more competitive in a program that emphasizes behavioral econ, at the opportunity cost of making yourself less competitive in a more traditional econ program, so make sure this is the field you really want to focus on.
You will still need the math.  So if you've studied multivariable calculus, differential equations, and real analysis anyway, then you've probably cleared many of the important bars.  If this MS you're considering would come at the cost of taking those core econ PhD pre-reqs, then that would be a problem.  Not necessarily an insurmountable one, however, if you can figure out how to fit those classes in.