Rangers needed to beat fellow strugglers Stoke on Saturday to stand any realistic chance of the greatest of escapes but instead fell to a 2-0 defeat at Loftus Road.
Peter Crouch did the damage against his former club, opening the scoring in the first half before winning the penalty from which Jon Walters made sure of the three points.
The defeat leaves QPR 10 points adrift of safety with only four matches remaining - a position Redknapp does not expect his side to recover from.
"It looks like that's it," the Rangers boss said. "It is almost impossible now.
"It is a big disappointment but even today [Saturday] I felt in the first half we were bang in the game.
"I was disappointed to come in at half-time 1-0 down as I thought we did okay in the first half.
"But we lack a forward ball and without Bobby Zamora it is very difficult. We've got no real target up there.
"Loic Remy needs to play off somebody as he is not a target man and it was just a problem for us.
"In the first half I thought Adel Taarabt in the hole was a threat to them and then we lose him to a shoulder injury after Andros Townsend does his hamstring. I felt the threat really disappeared then.
"I thought we would still be in with a chance at this stage when I came in, but you don't know until you get here and work with the team.
"You're not sure what you've got, who is who and how they are.
"But I felt we've been in every game apart from Liverpool, who battered us here and were different class.
"Other than that, we've been in every game. It is just that little dividing line that makes the difference."
Redknapp pinpointed Zamora's current suspension, injuries and the Loftus Road pitch as key contributing factors to QPR's impending relegation - something he was confident of avoiding when he replaced Mark Hughes in January.
"It is disappointing but I've had other disappointments and I'll have more in the future I am sure," Redknapp, who was last relegated eight years ago with Southampton, said.
"I get as disappointed as anyone when it doesn't go well but you can only do what you can do.
"I can't look back and say I have not worked hard and tried hard because I have. I have given everything and at the end of the day it just hasn't been good enough.
"We've got to keep going, we've got to keep playing. We need to make sure we prepare right this week to play Reading next week.
"You owe that to the supporters. They are going to travel to Reading and want to see a performance next week. That is what we have got to try and do."
While QPR's dalliance with the Barclays Premier League will soon be over, Stoke hope today's result will act as a catalyst.
The victory ends a six-match losing streak away from home and propels them six points clear of the drop zone.
"We just have to keep going after that," said Stoke assistant manager Dave Kemp, speaking instead of boss Tony Pulis.
"We've got four more games and we need to concentrate on our performance and getting more points.
"We're certainly not going to sit back and rest on that now. We've had a bad run and obviously came out of it today with the points so we've got to keep that going in the last four games.
"People asked if it was a must-win game but, no, it wasn't. A must-win game is the last day when you need three points.
"It was more a like-to-win game and we've won so our situation is better now than it was before the game.
"We've got Norwich at home next week and it gives us a boost - confidence is a big thing in football.
"We haven't played that badly away from home actually, although our results don't suggest that.
"We've played better than that and got beat. It was important to get the win today and hopefully we can take that on now."