Blue chips stocks struggled for direction amid confirmation that Prime Minister Theresa May is to officially notify the European Union next Wednesday that the UK is leaving.
Downing Street said she would write a letter to the European Council, adding that it hoped negotiations on the terms of exit and future relations could then begin as quickly as possible.
If all goes according to the two-year negotiations allowed for in the official timetable, Brexit should happen in March 2019.
The FTSE 100 Index edged 4.3 points lower to 7420.6, after top flight shares hit consecutive all-time highs on the final two days of last week. The FTSE 250 Index was down 7.8 points at 19087.1.
Miners were down reflecting easing expectations for growth in world trade, with Antofagasta down 11.5p to 841p and Anglo American 15.5p lower at 1277p.
IG market analyst Joshua Mahony said: "A somewhat slow start to the week today has seen the FTSE in the red from the get-go, with investors focusing heavily on commodities."
The biggest afternoon risers in the FTSE 100 Index were Dixons Carphone (+6.6p to 308.3p), Associated British Foods (+47p to 2663p), Old Mutual (+3.6p to 225p), Direct Line (+4.6p to 339p) and Sainsbury's (+3.7p to 273.4p).
The biggest afternoon fallers in the FTSE 100 Index were Hikma Pharmaceuticals (-41p to 2127p), Royal Bank of Scotland (-3.7p to 240.2p), Antofagasta (-11.5p to 841p), Convatec Group (-3.2p to 257.6p) and Anglo American (-15.5p to 1277p).
The biggest afternoon risers in the FTSE 250 Index were Evraz (+12.6p to 231p), Supergroup (+60p to 1538p), Vectura Group (+3.7p to 157.2p), Ladbrokes Coral (+2.8p to 132.2p) and Electrocomponents (+10p to 487.4p).
The biggest afternoon fallers in the FTSE 250 Index were Nostrum Oil & Gas (-24.4p to 445.5p), IP Group (-6.4p to 162p), JRP Group (-5p to 138.2p), Hill & Smith Holdings (-39p to 1313p) and Cranswick (-67p to 2423p).