Mobility allowance tax free or not?

Dear all, I recently started my fellowship (in fact I just got my contract to sign_and I am confused about the "Mobility allowance". There is a big debate whether this allowance is taxable or not. Is it true that at some universities you pay tax on the "mobility allowance", while for others it is tax-free? I don't really know the UK legislation about this so I would really appreciate your opinion. Thank you very much