The UK is thought to have slipped behind Brazil in the world pecking order after official figures showed the emerging market's economy grew 2.7% last year.
Brazil's total output in 2011, at an average exchange rate and at current market prices, was worth 2.469 trillion US dollars (£1.56 trillion), compared with Britain's total output of 2.42 trillion US dollars (£1.53 trillion), think-tank Cebr said.
This would see the UK fall one place to the seventh largest economy as Brazil leapfrogs to sixth position - and closes in on France and Germany.
Tim Ohlenburg, senior economist at Cebr, said: "It's a natural enough movement. Brazil has a large population, has massive natural resources and strong local industry. It's just a country set for higher growth."
Mr Ohlenburg added it was unlikely that the UK would overtake France or Germany in the rankings in the future.
Brazil's level of growth still represented a severe slowdown on the previous year's 7.5% as soaring business costs and uncompetitive industries took their toll on the country's performance.
Brazil has a population of about 200 million, more than three times the population of the UK.
The biggest drag on Brazil's economy continues to be industrial production, which shrank 0.5% in the fourth quarter compared with the previous quarter.
Manufacturers have blamed most of their problems on Brazil's currency, which has strengthened about 40% since the depths of the financial crisis in 2009, and 6% this year.