What if a British Catholic got elected to and accepted the papacy? Could he still remain a legal citizen of the United Kingdom, or would he forfeit his status? If forfeiture was not automatic, would he nevertheless be obliged to renounce his British citizenship?

WOULD it actually be possible to remain a subject of the reigning monarch, while at the same time being the sovereign of a foreign state -- i.e. Vatican City?

The reason for my asking is the sticky legal question that has just risen around the recent papal conclave, where an American cardinal got elected as the supreme pontiff of the Catholic Church. Pope Leo XIV has not forfeited his Peruvian citizenship; but his U.S. citizenship appears to be in jeopardy. Either he has automatically forfeited it or he will be pressured to renounce it.