Oil giant Royal Dutch Shell has replaced US retail group Wal-Mart as the world's largest company, the latest annual survey by Fortune magazine says.

There are also fewer US corporations in the list of the 500 biggest firms than at any time since the publication started keeping records in 1995.

The annual survey ranks the largest companies by revenues, not profits.

The new list gives ammunition to those who believe the US's top dog status in the global economy is under threat.

For the first time in 10 years, the biggest firm is not from the US.