Centrica shares suffered another blow today after a leading City broker issued a sell note on the British Gas owner just two days before its annual results.

The blue-chip stock has lost a fifth of its value since October as it faces up to Labour leader Ed Miliband's threat to freeze prices if he wins the next election and Energy minister Ed Davey's recent questioning of its gas retail margin.

Ahead of results on Thursday, which are expected to show flat operating profits for 2013, UBS has weighed in with a sell rating based on expectations that Centrica's retail margins will move to 4% in order to reduce political risk.

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The company's shares were today more than 2% lower, off 7.7p at 310.7p.