Britain's immigration minister, already under fire for what critics said was an overly harsh crackdown on undocumented immigrants, resigned Saturday — after it was learned that his housekeeper was an undocumented immigrant. Mark Harper, 43, was widely criticized last year for a government advertising campaign that sent mobile billboards into minority neighborhoods warning undocumented immigrants to "go home or face arrest." In his resignation letter, Harper contended that he never broke the letter of any law, saying he ran numerous background checks on the woman he hired as a housekeeper in 2007.