Top-flight shares slipped into negative territory after official figures showed inflation jumped to a greater-than-expected 0.5% in June, as rising air fares and petrol prices pushed up the cost of living.
The Consumer Price Index (CPI) measure of inflation lifted from 0.3% in May to its highest level for 20 months, according to the Office for National Statistics (ONS).
The FTSE 100 Index edged down 11.3 points to 6684.15 in afternoon trading, as the ONS revealed that the cost of plane flights jumped by a record 10.9% between May and June, partly due to the Euro 2016 football tournament in France. The FTSE 250 Index was up one point to 16868.8.
Mining stocks were hit as traders' prospects for world trade dimmed against a backdrop of political turmoil in Turkey.
Rio Tinto fell 87p to 2375.5p, Glencore was 6p lower at 180.4p and Antofagasta slipped 13.4p to 489.6p.
Trustnet Direct market analyst Tony Cross added: "Miners are languishing somewhat after a lacklustre quarterly production report for Rio Tinto kicked off the sector's reporting season."
In afternoon trading the biggest risers in the FTSE 100 Index were Coca-Cola HBC (+47p to 1568p), Sky (+16p to 896p), Land Securities (+18p to 1060p), Carnival (+55p to 3548p) and Sage Group (+9.5p to 677.5p).
The biggest fallers in the FTSE 100 Index were Rio Tinto (-87p to 2375.5p), Glencore (-6p to 180.4p), Antofagasta (-13.4p to 489.6p), EasyJet (-30p to 1108p) and BHP Billiton (-25.1p to 951.7p).
In afternoon trading the biggest risers in the FTSE 250 Index were Ascential (+8.9p to 254.7p), Fidessa Group (+76p to 2433p), Micro Focus International (+51p to 1883p), Dairy Crest Group (+15.5p to 587p) and Domino's Pizza Group (+9.2p to 373.5p).
The biggest fallers in the FTSE 250 Index were Cairn Energy (-9.5p to 190.8p), SIG (-4.2p to 106p), Weir Group (-52p to 1497p), Evraz (-5.4p to 157p) and OneSavings Bank (-6.9p to 208.7p).