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The FTSE 100 added 0.1% and the FTSE 250 rose 0.2% by 0707 GMT.
Oil major Shell continued its recent rally and advanced for the seventh straight session, while BP was also a major plus for the blue-chip index.
Small-cap Nostrum Oil & Gas jumped 5% after the Kazakh-focussed oil firm said it was evaluating options including a sale of the company.