Apple has overtaken Google as the world's 'most valuable brand', according to Millward Brown.

According to the research firm's 2011 BrandZ study, the rise in popularity of tablets and smartphones helped Apple topple the search engine from the top spot, which it held for four years. Apple has an estimated brand value of more than $153bn: that's an 84 percent increase on last year and a rise of 859 percent since 2006, when the BrandZ report first started.

"By nurturing its brand and constantly innovating, Apple is able to command a high price premium and weather economic turbulence, providing a global business success story that other brands can learn from," said Eileen Campbell, global CEO of Millward Brown.

Google's brand value slipped by two percent this year, which Millward Brown attributed to investments in the Google Android mobile operating system and its Chrome browser.

However, Campbell told Advertising Age: "Google is still pretty hot. It remains one of the most desirable and trusted brands we track."

Facebook was named the fastest growing brand, with its value rising by 246 percent on last year to $19.1bn, putting it at 35 in the list. Meanwhile, Baidu, the Chinese rival to Google, was the second-fastest growing brand, with a value 141 percent up on last year. Worth an estimated $22.6bn, Baidu was placed 29th in the list, and Campbell speculated that its increase was helped by Google's exit from China.

Furthermore, e-tailer Amazon's brand value rose by 37 percent to $37.6bn, ensuring it edged out US supermarket chain Walmart as the most valuable retail brand.

"Amazon benefits incredibly by having user reviews integrated into its site," said Campbell. "Everybody does that now, but Amazon was the first, so it's done an incredibly good job of building trust."

Apple and Google were joined by IBM and Microsoft, with McDonalds the only non-tech company in the top five.