Blue chip shares struggled for direction as traders fear another round of tension surrounding Greece's ongoing debt crisis.
The FTSE 100 Index lifted 24.1 points to 7213 in afternoon trading, as the International Monetary Fund, Germany and Greece argue over the latest terms of the southern European state's €86bn ($92bn, £73bn) bailout. The FTSE 250 Index edged up 2.2 points to 18608.2.
CMC Markets chief market analyst Michael Hewson said: "European markets have traded sideways this morning, as investors look for direction in an uncertain environment. Matters are being muddied by concerns of a re-escalation in the Greek debt crisis."
Germany's Dax and France's Cac 40 were both slightly higher.
In London, Thomas Cook was the biggest faller in the second tier after the holiday group reported a loss of £49.1m in the three months to December, despite seeing rising demand for holidays in Greece.
The firm said it was "cautious" about its prospects for this year, sending shares down more than 6%, or 6p to 86.2p.
The biggest afternoon risers in the FTSE 100 Index were Barclays (+3.4p to 228.4p), BP (+6.8p to 462.8p), Legal & General (+3.5p to 241.7p), Land Securities (+15p to 1041p) and Prudential (+22p to 1589.5p).
The biggest afternoon fallers in the FTSE 100 Index were Anglo American (-33.5p to 1297.5p), Smith & Nephew (-29p to 1172p), Antofagasta (-17p to 819.5p), Whitbread (-82p to 3965p) and Smurfit Kappa Group (-38p to 2122p).
The biggest afternoon risers in the FTSE 250 Index were Ashmore Group (+19.1p to 338.1p), Pennon Group (+29.5p to 846p), OneSavings Bank (+9.7p to 357.8p), Bankers Investment Trust (+18.5p to 735p) and Shawbrook Group (+6p to 258.8p).
The biggest afternoon fallers in the FTSE 250 Index were Thomas Cook (-6p to 86.2p), RPC Group (-59p to 1000p), Henderson Group (-5.1p to 211.8p), Dairy Crest (-13.5p to 603p) and Vedanta Resources (-21p to 1030p).