Sterling slide boosts FTSE 100, discounted rights issue knocks M&S

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* FTSE 100 up 0.6%, FTSE 250 up 0.1%

* M&S slips after discounted offering

* Exporters top boost to main index

* Pets at Home jumps after upbeat outlook

May 22 (Reuters) - The exporter-heavy FTSE 100 index gained on the back of a weaker pound on Wednesday as lawmakers signalled they would not back Prime Minister Theresa May's latest Brexit compromise, while Marks & Spencer slumped after a discounted rights issue.

The main index, whose companies get more than two-thirds of their profit from abroad, advanced 0.6% by 0727 GMT while the more domestically-focussed FTSE 250 was up 0.1%.

Internationally-exposed companies British American Tobacco , Unilever and Diageo were among the biggest support to the blue-chip index.

Blue-chip housebuilders including Persimmon and Barratt, which gained in the last session when May unveiled her new Brexit deal, gave up more than 2%.

M&S slid 5% to a more than four-month low after it priced a rights issue at a massive discount to Tuesday's close. It also reported a third straight decline in annual profit, emphasising the pain of its latest attempt at a turnaround.

Pets at Home jumped 8.4% on the midcap index after it forecast higher earnings for 2020. (Reporting by Muvija M and Shashwat Awasthi in Bengaluru Editing by Andrew Heavens)