Pharmaceuticals giant GlaxoSmithKline has come under pressure after the US drug regulator said it may approve a generic version of GSK's biggest selling product next year.
The US Food and Drug Administration has accepted a submission from rival GSK's rival Mylan for a cheaper copy of its asthma drug Advair.
Once an application has been submitted it is rare that it will not be approved, as they are typically copies of drugs that have been on the market for a number of years.
If the US watchdog does approve a generic of the drug it could cripple GSK's sales of Advair, which were last year worth £3.7bn.
While sales of Advair have dropped in the last few years – because insurance companies have forced prices down – the drug has been key to GSK over the last 10 years.
The news puts further pressure on GSK from the “patent cliff”, where big pharma companies are losing patents for some of their biggest money making drugs.
The announcement will turn up the heat on GSK's chief executive Sir Andrew Witty to develop new drugs to deal with the potential shortfall. The company has previously said it will make £6bn from sales of new drugs by 2018.
And that's after star fund manager Neil Woodford, who has held GSK shares for some time, last month said the company should undergo a radical restructure.
Earlier this month GSK reported earnings for 2015 up four per cent on 2014 at £23.9bn, with a profit of £10.5bn.