England manager Martin Johnson described his selection for the Calcutta Cup match with Scotland as the toughest of his tenure.

The "marathon" deliberations following England's 20-16 defeat to Ireland resulted in two key decisions - to retain Delon Armitage at full-back ahead of Ben Foden and replace flanker Lewis Moody with Joe Worsley.

"It was probably the longest selection process since I have been involved and the toughest but that's good. There is pressure on players to perform," said Johnson.

Armitage knows that more than most. The London Irish full-back has been struggling for form all tournament and did not expect to be selected, even after proving he had recovered from a rib injury.

Foden replaced him off the bench against Ireland and made an immediate impact, enhancing his already powerful claims to be awarded a first Test start.

Johnson left the final decision until this morning but eventually decided to stick by Armitage for England's trip to Murrayfield and use Foden as an impact player off the bench.

But Armitage knows his form must improve against Scotland or the full-back decision for England's final RBS 6 Nations game against France will be much easier for Johnson to make.

Armitage said: "It was a massive relief to be named in the team. I kind of expected not to be in it.

"I haven't had the best Six Nations and after going off injured Ben came on and did a good job off the bench.

"For me to get another shot to try and get back to the form I showed last year is a good opportunity.

"But obviously I need to start showing something. Foden is right there behind, he's been playing really well for his club, and the management and coaching set-up know he's ready.

"I could almost say I'm on my last wheels, this is my last chance.

"I need to start showing some improvement because I think I just haven't been playing well enough.

"I probably have been too impatient but I need to get into form as soon as possible because I don't want give that 15 shirt up. I know Ben is right there.

"If I don't show some improvement I think I'll lose the shirt very soon. But I know there is a performance in me.

"I'm not going to be negative. I'm a confident player and I have a job to do for the boys. I want to do it to the best of my ability and I will do everything I can."

Armitage had only just recovered from shoulder surgery by the start of the Six Nations and England's attack coach Brian Smith acknowledged he is not at his best.

"We are happy with what he is doing but there is more to come from Delo," said Smith. The gauntlet has been laid down.

Johnson, who spent his whole playing career at Leicester, laughed off the tongue-in-cheek suggestion that Moody was dropped as punishment because he is leaving Welford Road for Bath next season.

With his serious face on, Johnson insisted Moody had not been dropped at all - rather that Worsley had been preferred at openside for a physical showdown with the Scottish back row.

Worsley is widely considered to be the best defender in the England set-up, he is a powerful ball-carrier and Johnson believes the Wasps veteran is the perfect fit to tackle the Scots.

"We had this conversation last year when we picked Joe at seven but he was probably our best player," said Johnson.

"There is not a weakness in Lewis' game but Joe's defence is good and his ball-carrying is particularly strong."

Johnson's other change to the side from the Ireland game was the expected inclusion of Leicester lock Louis Deacon in place of the injured Simon Shaw.

But there was a surprise on the bench, with Leicester's uncapped scrum-half Ben Youngs preferred to Paul Hodgson.

Youngs, 20, went head-to-head with Hodgson in Leicester's 35-19 victory over London Irish on Saturday and, like Armitage, he was also surprised to be selected.

"It is a harsh call on Paul Hodgson who has been fantastic for us on and off the field. Ben is playing well enough and we feel he deserves a chance," said Johnson.

"I told Ben he was on the bench and he said 'but Leicester haven't got a game this weekend'.

"I had to say 'no, you are on the bench for us!'"

Murrayfield has been a graveyard for England in recent years. Ben Youngs' father, Nick, won six caps for England at scrum-half and lost there in 1984.

Saturday's game will mark the 20th anniversary of Scotland's 13-7 Grand Slam-clinching victory in 1990 and England have been beaten on each of their last two visits to Scotland.

Adding extra spice to the occasion, Johnson goes up against the former England coach Andy Robinson, who is now in charge of a Scotland side smarting from three straight defeats.

"They will be very confrontational but they are not afraid to try things so you have to be very alert," said Johnson.

"Their driving game has improved since he has been there. It will be a typical England-Scotland encounter in many ways."