Food outlets in a part of south-east London have the worst hygiene standards of more than 2,000 postcodes in England, Northern Ireland and Wales, an investigation has found.
Bexley had six of the 10 worst postcodes studied for food hygiene.
But in one Birmingham postcode not one outlet received below a "generally satisfactory" score.
Consumer watchdog Which? compared Food Standards Agency hygiene ratings from January 2011 to March this year.
It found wide variations between different areas and High Street chains.
In the worst postcode, DA7, Bexley, nearly half of the eateries inspected (45%) rated lower than "generally satisfactory", the watchdog found.
B35, the Castle Vale district of Birmingham, offered the highest standards of food hygiene.
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."
Across England, Northern Ireland and Wales, Which? found 18% of La Tasca outlets inspected had a rating of less than "generally satisfactory", while 13% of Little Chefs inspected had low ratings.
About three in 10 Chicken Cottage outlets (29%) and a quarter (26%) of Dixy Chicken takeaway shops inspected were rated below "generally satisfactory".
A spokeswoman for La Tasca said the firm disputed the data and the survey was not an accurate reflection.
She said it has not received any enforcement notices in the UK in the past 18 months.
A number of convenience store chains also had about 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 research suggested 75% of consumers would not eat at a food outlet that received a hygiene rating below "generally satisfactory".
Which? surveyed 2,000 people between 28 and 31 March.