LONDON—Britain and Iran issued warnings to each other’s diplomats Monday as tensions escalated over a recent attack on an oil tanker in the Arabian Sea. Britain began the day by summoning Iran’s ambassador, Mohsen Baharvand, to the Foreign, Commonwealth & Development Office in response to Thursday’s drone strike on the MV Mercer Street, which killed a British national and a Romanian. Minister James Cleverly, who oversees Middle East affairs, “reiterated that Iran must immediately cease actions that risk international peace and security, and reinforced that vessels must be allowed to navigate freely in accordance with international law,” the Foreign Office said in a statement. The United States, Britain, and Israel have blamed Iran for the fatal attack, though Iran denies involvement. Calling it an “unlawful and callous attack,” British Foreign Secretary Dominic Raab said Sunday that the UK and its allies were planning a coordinated response. Iran’s Foreign Ministry on …