A steely determination and a good wife are behind Manchester United manager Alex Ferguson's biggest achievement of staying in the job for 25 years, according to his son Darren, who thinks his father will continue for three more years at least.

The Scot has guided the club to 12 English league titles, two European Cups and five FA Cups among others in a quarter of a century, which he marks on Sunday, but it is his longevity that fellow manager Ferguson junior is most impressed with.

To me, 25 years in any job, let alone being the manager of Manchester United, is a fantastic achievement, the younger Ferguson, who is in charge of Championship (second division) Peterborough United, told Reuters in an interview on Thursday.

That's got to be the main achievement he's done -- I know he's won all the trophies but the length of time and the amount of teams he's built is his biggest achievement no doubt.

Ferguson's son knows all about clubs being quick to change bosses in an era of impatience for success -- having spent less than a year at the helm of Preston North End last year -- and said there was no chance his dad expected to last this long.

Ferguson senior made a slow start at Old Trafford and was reportedly on the verge of being sacked in December 1989 after a poor run but the club kept the faith -- where many would not these days -- and their fortunes changed.

Ten years later he had celebrated a treble of Champions League, Premier League and FA Cup, and 22 years later he had taken United past arch rivals Liverpool's record of English league titles with a 19th.

All his success means he can provide valuable advice to the son who was inspired to follow his choice of career -- even if not all of it is welcome.

There's times when he's like any father ... there's times when I'll speak to him and disagree with him, said Darren, who came through the United youth ranks before making 27 first team appearances in the early 1990s.

However, the 39-year-old has listened to what he says was his dad's top tip.

Just be honest with your players, regardless of whether it's good stuff or bad stuff or you're going to upset them, he explained. You'll get their respect if you are honest with them.

MORE TROPHIES

Ferguson might be like a lot of dads in some respects but his son also remembers other parts of his childhood that were different.

He won't mind me saying this -- my mother brought us up, he wasn't there a lot because he worked hard, he said.

I have to say there's no way he would have the success he has without my mother -- there is no doubt about that.

With his 70th birthday looming next month, the Manchester United boss is showing no signs of winding down and his son believes he has several more years in the management tank thanks to the qualities that set him apart from others.

His determination and desire to keep on churning out teams is the thing that's made him different to anyone else, he said. He still gets up early, gets into work early, probably before most people.

I think he'll stay another three years at least. I just think the team he's creating now ... Teams go in cycles and it'll be another three years maybe more.

More years means more trophies for the cabinet.

Definitely, yes, Darren said. The Champions League is a possibility, the league is a possibility. With Manchester United, anything is possible.

(Editing by Mark Meadows)