This was on BA's website:

Additional security measures for US flights

Summary
The United States government has revised its security arrangements for all customers travelling into the US.

Please be aware that due to these revised security arrangements for all customers departing on a flight from Heathrow or Gatwick to the US only one item of hand luggage is allowed. They are advised to check-in as normal.

Customers travelling to other destinations outside the United States are not affected.

Please check ba.com for further updates during the day.




More information
What if I have wrapped presents in my hand luggage?
Due to additional screening of all US-bound passengers and hand luggage before they board their flights, we would advise passengers to check in all wrapped items at the airport. Please be aware that passengers will be required to unwrap any items at the gate if they have chosen not to check them in.

Will passengers be charged for checking in their extra hand luggage?
There will be no charge for checking in extra items of hand luggage.

What airports are affected?
Passengers travelling to the US from Heathrow or Gatwick are advised that due to additional security screening they are only allowed to take one item of hand luggage on board.

Customers are advised to check the status of their flight before they leave for the airport.