After reports that thousands of UK travellers could be turned away at the US border, Consul General Derwood Staeben said that was "alarmist" and the new system aimed "to encourage travel to the US".
The Foreign Office had said the forms must be completed at least 72 hours before departure, but Mr Staeben said that was incorrect.
"You can fill it in from an internet cafe in the airport if you want to, although obviously we do advise people to do so as far ahead of their journey as possible," he said.
Mr Staeben said there was no limit to how far in advance the form could be completed and insisted that rumours of thousands of people being turned away were incorrect.
"We are implementing this in a reasonable way," he said.
"People who don't do it can expect to be delayed, denied boarding or turned away - that is technically true - but we're working with the travelling public on this.
"We will take a reasonable approach. We want to encourage travel to the US."