LONDON  — Britain faces the prospect of an immediate and prolonged Recession, spiking unemployment and inflation if it crashes out of the European Union in March with no deal on future relations, a leading credit ratings agency warned Tuesday.

In a wide-ranging report, Standard & Poor’s said a “no-deal Brexit could push the U.K. economy into a moderate recession and lower the economy’s long-term growth potential.” In 2019, for example, it predicts the British economy will shrink 1.2 percent if no deal is agreed, against the growth of 1.3 percent that it expects if a deal is reached.