The arch-Brexiteer said: "In 159 days, the United Kingdom will leave the European Union.
"As the Brexit negotiations enter the final stage, we must stand firm, keep the nerve and monitor the prize, a good agreement with our EU partners."
Raab said that the option to extend the transition period, suggested by the Prime Minister, beyond December 2020, would only be an "alternative" to the support that the bloc demands.
Mr. Raab said: "The Prime Minister has rightly refused to rule out the possibility of considering different approaches, including extending the implementation period for a limited period of a few months, as an alternative to support.
"But we are not going to sacrifice Northern Ireland, and we must have a definitive goal, either through a time limit or a mechanism that allows the United Kingdom to leave, in case the EU does not fulfill its promise to obtain the Future relationship in place quickly.
"It's understandable that people want to know when the Brexit process will end."
The intervention of the Brexit Secretary will be welcomed by the Brexiteers, who were furious when Ms. May offered to extend the transition period "for a matter of months", as well as to agree on a backup.
The prime minister shared her latest plans during the second day of the European Council summit last week.
She said: "Another idea that has come up, and is an idea at this stage, is to create an option to extend the implementation period by a matter of months, and it would only be a few months.
"But the point is that it is not expected to be used, because we are working to ensure that we have that future relationship by the end of December 2020."
"I know it's possible to do it and that's what we're working for.
"Under those circumstances, there would be no need for any such proposal and I am clear that I expect the implementation period to end at the end of December 2020."
Brussels has demanded that support be necessary to protect Northern Ireland from a difficult frontier if negotiations for a future relationship with the 28-nation bloc are broken.
The suggestion that the back-up mechanism could be completely eliminated could enrage the EU negotiators.
But Raab remains confident that the UK can come to a good agreement.
He wrote in the Sunday Telegraph: "At the same time, our planning and preparation without agreement will continue.
"We are building on the 106 technical notices that we have published that explain how we will avoid, manage or mitigate the short-term risks of a scenario without an agreement, and we will achieve success in Brexit."