How long do you need to work to earn enough money to buy yourself a Big Mac?

The average Londoner makes enough in just 13 minutes to head off to McDonald's. A New Yorker takes one minute longer, a Parisian needs to work for 20 minutes and a worker in the Polish capital Warsaw would have to work for just over half an hour.

But that is nothing compared to Nairobi, where the Big Mac requires a staggering two hours and 38 minutes of labour.

Every three years since 1971 Swiss banking giant UBS has calculated the relative pricing and earnings power in more than 70 capital and top cities around the world. Today it has enhanced its famous Big Mac index with an iPod Nano index.

This shows a worker in Zurich or New York needs to work just nine hours if he wants to enjoy eight gigabytes of music while he munches his burger. In London the same piece of kit requires 11 hours of work.

In Paris and Nicosia it takes 15 hours while in Mumbai a worker would have to put in virtually 20 nine-hour days (177 hours), or a month's wages, to afford an iPod Nano.

London, which was the second-most expensive city to live in in the 2006 survey, plunged to 21st place this year as the pound collapsed against the dollar when the data were collected in March and April.

The pound was worth just $1.40 at its lowest point then. At today's $1.65 level London moves back up to around fifth place.