Русско-англо-русский юридический словарь

When one obeys the letter of the law but not the spirit, one is obeying the literal interpretation of the words (the "letter") of the law, but not necessarily the intent of those who wrote the law. (Wikipedia)

By the letter of the law, Belfast Judge Isobel Brownlie may well have been correct to find a Christian-run bakery acted unlawfully when it refused a gay man’s request for a cake carrying a slogan that promoted same-sex marriage. (dailymail.co.uk)