In August 2013, the rules for using the Fast Track security lanes at Heathrow were changed. If you were flying on British Airways, American Airlines or Iberia, you were allowed to use Fast Track if you held a Sapphire-level (mid tier) status card from any oneworld alliance airline.
It is worth noting that this was a special benefit arranged by British Airways for itself and its key partners at Heathrow and was not general oneworld policy.
I have received two emails in the last couple of weeks from readers which makes me think that this policy has been changed. Both readers had been turned away from the Fast Track lane in Terminal 5 when they flashed their Cathay Pacific Gold card.
If you are a oneworld Emerald (ie top tier) cardholder then nothing has changed. You will still get Fast Track security at Heathrow Terminal 1, 3 or 5 when flying with specific oneworld airlines. This is outlined on the oneworld website here. Terminal 4 is not listed for some reason which is unfortunate if you are flying with Qatar etc. For some reason Terminal 5 only lists British Airways as a participating airline but I assume Iberia flyers would be OK as well.
The impact of this change is ONLY on oneworld Sapphire cardholders who must now join the general line.
I imagine that this is a cost cutting measure by British Airways as Heathrow makes an additional charge for every passenger who uses the Fast Track line who is not travelling in Business or First Class.
Oddly, although I cannot find the exact page now, it is possible to download the contract the airlines and American Express (for Centurion and Corporate Platinum cardholders) must sign from the Heathrow website!