The distance from the Grosvenor Hotel to the Dorchester Hotel is less than half a mile. The last time we met for dinner, Harry Redknapp had to get a taxi.

He started off with the best intentions for a short stroll, but retired hurt no more than 50 yards down Park Lane. This was before his left knee went, too.

Back then, it was just the right that had him in agony. His condition has deteriorated greatly.

Harry Redknapp quit QPR on Tuesday with the club second bottom of the Premier League

Redknapp says his knee problem was the main reason why he had to quit QPR as he could no longer walk

Redknapp outside his home in Dorset

On several occasions over the last two years, Redknapp has considered resigning from football management. He misses the hands-on engagement of coaching, he misses getting out onto the training field as he used to.

He still scouts games, but it's a chore. He has to make plans for staircases, for distant car parks; if he's on the wrong side of the stadium, it can take him 20 minutes to get to his seat. Sooner or later, something had to give.

He quit Queens Park Rangers on Tuesday, after being told he would need replacements for both right and left knees. Unable to walk even 100 yards, he accepted he was not the man to lead Rangers into their relegation fight. And he called owner Tony Fernandes in the small hours to deliver the news, shortly before sharing the decision with his son, Jamie.

'I was awake all night, thinking about it,' Redknapp told me. 'I'm struggling so badly now. I can't walk, I can barely stand and watch. I'm in pain all the time. I've been putting it off, and putting it off, but it has got to the stage where I cannot do the job.

'I booked a ticket for Fulham's FA Cup game with Sunderland on Tuesday night, but my first thought was, "how am I going to get to the ground?" Even if I get a car park pass there is going to be some walking. I can't walk around Craven Cottage anymore, I can't walk down the street – that's how bad it has got.

'I went to see my grandson play football at the weekend, and after five minutes had to go back to the car. I couldn't even stand up. What sort of life is it if you can't watch the kids play? That sort of made my mind up.

'I went to bed thinking I would sleep on it, but then I couldn't sleep a wink. That's when I decided to call Tony. It must have been 5.30am. I just told him he needed someone who could properly coach and manage the team in the next ten weeks. It's such an important time. They need someone who can give it everything.'

Redknapp says he had a good relationship with QPR owner Tony Fernandes

Harry Redknapp guided QPR back to the Premier League at the first attempt via the play-offs

Redknapp says Emmanuel Adebayor was his only transfer target on deadline day - but he was too expensive

Redknapp's resignation immediately brought speculation that he had become frustrated at a lack of activity in the transfer window. Rangers face financial fair play restrictions, and a huge fine, and have been working to cut budgets.

Yet he insisted this was not the case. 'I haven't got the hump, we haven't had a row,' said Redknapp. 'I knew some while ago that we were not going to be able to get much done in January.

'We had one real target on the last day, Emmanuel Adebayor, because we are short upfront. But he was too much money. I accept that. There are no hard feelings on my part – I've not had a problem with Tony Fernandes in all my time there.

'I even went to training this [Tuesday] morning because it was my intention to say goodbye to the lads. I went to sort out some bits and pieces with the club and by the time I had finished they were gone. It's been that quick. I just made my mind up because events were piling up. Being told I needed both knees replaced was a huge blow. It would put me out of the game for months because you can't have them done together.

'Steve McClaren called me when he heard the news and we were talking about our time together at Rangers last year. He reminded me that I was on crutches for 10 weeks back then. It's been a problem for too long now.

'I know what people think - that I've been sacked, or stormed off because we couldn't get the players in.

'My son Jamie said that my timing has to be the worst in the world. When I look back on my career, it certainly isn't my strong point - but I can't control what people think. I feel positive about the future at Rangers - Sandro is back at the weekend and he will make a huge difference. We've got other players coming back from injury, too - if I could get out and coach them like I could five years ago, I'd be optimistic. But I can't.'

Redknapp and QPR have not won a single point away from home this season

His wife, Sandra, broke a knee cap too, before Christmas, stumbling over the vacuum cleaner. He regrets not being at home more to tend to her and you can sense a desire to be closer to home.

'I was totally honest with Tony. I told him he needed someone who could commit to every aspect of the job in the next ten weeks, and that's not me. I still don't think I'm finished with football. When I've had the operations, I'll be looking for work again, I know that. I can't imagine my life without it. But right now, I've got to make my health the priority.

'I can't even walk the dogs. I stand watching the team play, and I'm struggling. It's reached a point where I can't ignore it any longer. It's not a decision I would take lightly. I've thought about it a few times this season, but have always decided to give it one last go.

'The last week has been the tipping point. I can barely move and I'm not enjoying my life. I need to get this done as quickly as I can and try to move on. Nigel Pearson at Leicester City told me had a knee replacement and was ready to go again after six weeks. I'm not sure I'll be hopping about in that time, but I'll talk to Andy Williams, my surgeon, and we'll work out a plan.'