On the non-existence of singular Borcherds products

Let $l\ge 3$ and $F$ be a modular form of weight $l\mathrm{/}2-1$ on $\mathrm{O}(l,2)$ which vanishes only on rational quadratic divisors. We prove that $F$ has only simple zeros and that $F$ is anti-invariant under every reflection fixing a quadratic divisor in the zeros of $F$. In particular, $F$ is a reflective modular form. As a corollary, the existence of $F$ leads to $l\le 20$ or $l=26$, in which case $F$ equals the Borcherds form on ${\mathrm{I}\mathrm{I}}_{26,2}$. This answers a question posed by Borcherds in 1995.