Finance minister George Osborne also warned that a potential 'Brexit', or departure from the European Union, would risk damaging the nation's economic recovery, ahead of a key referendum in June.
Chancellor of the Exchequer Osborne said the government would seek additional spending cuts totalling 3.5 billion pounds (USD 5.0 billion, 4.5 billion euros) by 2020, when it expects to reach a budget surplus despite higher borrowing.
The chancellor pointed to a "dangerous cocktail of risks" including "turbulence in financial markets, slower growth in economies like China, and weak growth in the developed world" for the growth downgrades.
"How these cuts will be made has not been outlined other than described rather vaguely as savings," said ING economist James Knightley.
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"With (government) department budgets already having been cut aggressively it will be interesting to see where new efficiency savings can be made."
In a speech lasting around one hour, Osborne forecast that the British economy was set to grow by 2.0 per cent this year, down from a November estimate of 2.4 per cent.
Growth was expected to stand at 2.2 per cent next year, down from 2.5 per cent.
Osborne, a top figure in Prime Minister David Cameron's Conservative party and government, also revealed plans to cut several taxes levied on businesses amid strongly divergent views from companies on whether Britain should quit the EU.
"This is a budget for small businesses," Osborne told lawmakers.
There were also significant tax cuts for the oil and gas industry, which has been hit by tumbling energy prices.
Turning to Britain's June 23 referendum on EU membership, Osborne repeated the government's strong desire for the UK to stay within the 28-nation trading bloc.
The government will meanwhile plough more cash into education and infrastructure projects.
Osborne approved major railway developments in northern England and in London, and also unveiled a package of extra funding for education, which could see students being made to learn maths until the age of 18, up from 16.