Wires

FTSE 100 heads for fourth session of gains

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* FTSE 100 up 0.1%, FTSE 250 up 0.2%

* AstraZeneca biggest boost to main index

* RBS slides after reporting Q3 loss

Oct 24 (Reuters) - London's FTSE 100 held firm at a near one-month high on Thursday as gains in AstraZeneca after a 2019 target upgrade countered losses in Royal Bank of Scotland following a weak third-quarter update, while midcaps outperformed as sterling rose.

The blue-chip index inched up by 0.1% at 0708 GMT, while the FTSE 250 index gained 0.2%.

AstraZeneca added 2.3% after the pharmaceutical giant raised its annual product sales forecast for the second time this year, thanks in part to strong demand for new cancer treatments.

On the other end of the spectrum was Royal Bank of Scotland that skidded 3% as it swung to a third-quarter loss due to a fresh 900 million pound provision to settle mis-selling claims in Britain's biggest consumer banking scandal. (Reporting by Muvija M and Shashwat Awasthi in Bengaluru; Editing by Arun Koyyur)