É a primeira vez em que uma economia sulamericana ultrapassa a do Reino Unido.
Evento foi considerado como o resultado de uma grande mudança econômica mundial, acelerada pela crise europeia.
Até 2020, Brasil pode ultrapassar França e Alemanha. Estudo considera Rússia e Índia como economias mais dinâmicas no longo prazo.
Estudo é do Centro de Pesquisa Econômica e de Negócios (CEBR).
Abaixo, reportagem da BBC de Londres.
Brazilian economy overtakes UK's, says CEBR26 December 2011 Last updated at 12:25 GMT
Brazil has overtaken the UK as the world's sixth largest economy, an economic research group has said.
The Centre for Economics and Business Research (CEBR) said its latest World Economic League Table showed Asian countries moving up and European countries falling back.
The CEBR also predicted that the UK economy would overtake France by 2016.
It also said the eurozone economy would shrink 0.6% in 2012 "if the euro problem is solved", or 2% if it is not.
CEBR chief executive Douglas McWilliams told BBC Radio 4's Today programme that Brazil overtaking the UK was part of a growing trend.
"I think it's part of the big economic change, where not only are we seeing a shift from the west to the east, but we're also seeing that countries that produce vital commodities - food and energy and things like that - are doing very well and they're gradually climbing up the economic league table," he said.
A report based on International Monetary Fund data published earlier this year also said the Brazilian economy would overtake the UK in 2011.
Brazil has a population of about 200 million, more than three times the population of the UK.
Brazil's economy grew by 7.5% last year, but the government has cut its growth forecast for 2011 to 3.5% after the economy ground to a halt in the third quarter, with analysts blaming the country's high interest rates and the worsening situation in the eurozone.
And although Brazil currently sells more to China than it imports, Brazilian manufacturers have complained that their industries are being affected by cheap mass-produced goods from the Asian giant.
The CEBR also said that Russia moved up one spot in its league table to ninth in 2011, and predicted that it would rise to fourth spot by 2020.
It predicted that India, the world's 10th biggest economy in 2011, would become the fifth largest by 2020.
And it said European countries would drop down the table, with Germany falling from fourth in 2011 to seventh in 2020, the UK from seventh to eighth, and France from fifth to ninth.
CEBR World Economic League Table
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