The wait is nearly over for many passengers
A bottleneck of flights and passengers built up as planes were grounded in the wake of Tuesday's attacks on the World Trade Center and the Pentagon.
US airspace was closed and now carriers face tough new security regulations as well as knock-on delays.
The airport has been forced to adopt special arrangements to accommodate many passengers arriving up to six hours before departure.
At Terminal Four, where British Airways (BA) flights depart for the US, passengers are being held in a marquee outside departures while they wait their turn for check-in.
Many are huddled together under blankets.
Temporary home
All are being offered tea, coffee and refreshments from a mobile canteen.
At Terminal Three, where Virgin, United and American airlines flights depart for the US, the bottom storey of the NCP car park continues to act as a temporary home to hundreds of passengers.
Thousands more wait patiently in line in the main check-in area.
US carriers are running a near-normal service with BA and Virgin also returning to normal.
Regulations banning low-level flights over central London have been lifted, allowing aircraft using Heathrow and London City airports to resume their normal flight paths.
Extra security
Airlines are aiming to give priority, on compassionate grounds, to those trying to get back to the States to find out about loved ones caught up in the tragic events in New York and Washington.
Extra security measures mean those passengers who have been allocated to a flight face a longer than normal wait to check in.
There are also tighter than normal restrictions on items which can be carried in hand luggage with even small sharp objects such as razors and nail scissors being banned from the aircraft cabin.
The list includes toy or replica guns (plastic or metal), as well as household cutlery and knives with blades of any length.
Also banned are letter openers, tradesmen's tools, darts, knitting needles and sports goods such as rackets, cricket bats and golf clubs.
Passengers found with these items will be asked to place them in their hold luggage or have them confiscated if they have only hand luggage.