Wicketkeeper warns of hard work ahead to secure victory
Despite a first-innings lead of 188, England's first Test game plan was in danger of being blown away by a stunning spell from Ishant Sharma that left the hosts 62 for five.
Prior responded with a measured knock of 103 not out - his sixth Test hundred and third at Lord's - to steer England to a declaration on 269 for six and a lead of 457.
India had knocked 80 of those off at stumps for the loss of Abhinav Mukund and Prior remained wary of the tourists' stellar batting line-up, despite the world record target facing them in the morning.
"I think we're probably just ahead but there's still a lot of runs in that wicket," he said. "There are no devils in the wicket, it's still a pretty good deck. There are a huge amount of runs there, certainly when the ball gets a bit older. Graeme Swann's going to play a huge part for us tomorrow and there is a lot of hard work ahead.
"India's batting line-up is pretty strong and the ball might not be swinging around and seaming around like the first innings. We might go an hour without taking a wicket but hopefully we'll have one of those silly hours where we'll take three or four."
Prior admitted he was taken aback to be thrust into such a pressure situation in the morning session.
"I wasn't really expecting to be batting before lunch. I was looking at the menu and thinking 'I'll have the rack of lamb, that'll be nice' and then there I was taking guard," he said.
"I felt pretty nervous to be honest. But what's not to love about it (Lord's]. It's a flat deck, a quick outfield and the sun always shines on me whenever I walk out to bat. I made my debut here and I always love coming back."
Sharma, who returned four for 59 after going wicketless in the first two days, put his resurgence down to familiar issues with the Lord's slope.
"In the first innings I was not used to the slope, it is my first Test match at Lord's and I didn't know how to use it," he said. "I spoke to Zaheer (Khan] about it and he helped me a lot to learn about the slope."
India are hopeful Gautam Gambhir will be okay to bat in the morning after Prior struck him on the elbow with a powerful sweep. X-rays showed no break but he was unable to take his usual opening berth.
Sachin Tendulkar, meanwhile, could see his innings delayed after he missed two sessions in the field with a viral infection.