Food outlets in parts of a London borough have the worst hygiene standards out of more than 2,000 postcodes, according to a Which? investigation.
Bexley in south east London had six of the 10 worst postcodes studied, with the worst postcode, DA7, having nearly half of its eateries inspected (45%) rated as lower than "generally satisfactory", the watchdog found.
In contrast, none of the food outlets inspected in Birmingham's B35 area received a score below "generally satisfactory".
Which? looked at Food Standards Agency hygiene ratings in postcodes across England, Northern Ireland and Wales from January 2011 to March this year, finding wide variations between different areas and high street chains.
It found 18% of La Tasca outlets inspected had a rating of less than "generally satisfactory", while 13% of Little Chefs inspected had low ratings.
Three in 10 Chicken Cottage outlets (29%) and a quarter (26%) of Dixy Chicken takeaway shops inspected were rated below "generally satisfactory".
A number of convenience store chains also had around a fifth of their outlets inspected rated less than "generally satisfactory", including Best-In and Best-One (22%), Costcutter (21%), Premier (21%) and Londis (20%).
But some major chains had no poor scores recorded at all, including Carluccio's, Eat, Marriot Hotels, Premier Inn and Zizzi.
A survey to accompany the study found that 75% of consumers would not eat at a food outlet that received a hygiene rating below generally satisfactory.
Which? executive director Richard Lloyd said: "Our investigation shows that food hygiene is an unacceptable postcode lottery.
"Diners shouldn't be taking a risk with their health simply by choosing the wrong area in which to eat out.
"We want everywhere that serves food to the public to display their hygiene score prominently so people can make an informed choice."
Which? surveyed 2,000 members between March 28 and 31 for its investigation.