The threat of slowing demand in GlaxoSmithKline’s flagship HIV drug business prompted investors to dump the FTSE 100 stock, sending shares to an eight-week low in intraday trade.

US investment bank Citigroup dealt a blow to the blue chip stock when it downgraded its rating to “neutral” from “buy” and cut its earnings forecast by up to 9pc, as it warned the “HIV market boom years may be over”.

Analyst Andrew Baum also cautioned that the repeal of the Affordable Care Act, or Obamacare, could shrink the US HIV market by increasing the number of uninsured patients and negatively impact prices.