LONDON — In the United States, the debate over how to cut the long-term budget deficit is just getting under way.

But in Britain, one year into its own controversial austerity program to plug a gaping fiscal hole, the future is now. And for the moment, the early returns are less than promising.

Retail sales plunged 3.5 percent in March, the sharpest monthly downturn in Britain in 15 years. And a new report by the Center for Economic and Business Research, an independent research group based here, forecasts that real household income will fall by 2 percent this year. That would make Britain’s income squeeze the worst for two consecutive years since the 1930s.

All of which has challenged the view of Britain’s top economic official, George Osborne, that during a time of high deficits and economic weakness, the best approach is to aggressively attack the deficit first, through rapid-fire cuts aimed at the heart of Britain’s welfare state.