Following their heart-breaking 13-12 loss to Argentina, Scotland now have to beat England by their biggest margin in 25 years to stand any chance of qualifying for the quarter-finals.
But although England are unbeaten in Pool B, they know that losing to Scotland could also signal the end of their campaign.
"It is going to be massive, the biggest game of my career," said Armitage.
"This is a World Cup. You lose, you go home. You win you progress.
"They are very dangerous but we are in the same situation as them. We will be up for it.
"We know how Scotland will come at us. Every year they come and try and turn us over. We will be ready for it.
"We are not thinking about what might happen - we are just thinking about playing Scotland and trying to get the result.
"Our focus is going in there ready to play a Test match against a tough Scotland team who need to win. We have to be ready for it."
Armitage may have to settle for a place on the bench against Scotland despite impressing on the wing against Argentina and Georgia.
Mark Cueto marked his return to action against Romania with a hat-trick, while fellow winger Chris Ashton also scored a treble - but Armitage insists he will be working hard in training to give Martin Johnson a selection headache.
"I am very pleased with my form. There is still a lot more to come," said Armitage. "It was tough sitting on the bench against Romania, watching the two wings score a hat-trick and Ben Foden getting over the line.
"I have been working very hard in the games before that and hopefully I get a run.
"If not, as long as England are doing well, the guys are scoring tries and we are winning that is all that matters."
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