"We take these decisions looking at the evidence in front of us and working with our partners. This is something we have discussed with the Americans and what we've done is put in place some extra precautions and some extra checks," Cameron told ITN.
The U.S. Department of Homeland Security also beefed up security ahead of Independence Day -- a time when authorities are already on high alert for terrorist activity.
"You know the safety of the traveling public must come first. I hope this won't lead to unnecessary delays but it's very important that we always put safety first and we always do," said Cameron.