Lt Dalzell was a platoon commander who joined the Army in 2007
A British soldier killed on his 27th birthday in a major offensive against the Taliban in southern Afghanistan has been named by the Ministry of Defence.
Lt Douglas "Dougie" Dalzell, from Berkshire, of 1st Battalion Coldstream Guards, died in a blast in the Babaji area of Helmand province on Thursday.
Lt Dalzell's family described the platoon commander as a "true hero".
Three British soldiers have been killed in Operation Moshtarak, and 263 in total in Afghanistan since 2001.
'Leading from front'
A statement by Lt Dalzell's family said: "Our darling son and brother to Olivia and Angus, you touched the hearts of so many. You are our world and we love you with all our hearts.
"You only ever looked for the good in people and kept strong and happy in all you did. You are a true hero and we will miss you forever."
Lt Dalzell, from Hamstead Marshall, near Newbury, leaves parents Anthony and Colette, brother Angus and sister Olivia.
Lt Col Toby Gray, commanding officer, 1st Battalion Coldstream Guards, said Lt Dalzell was "assured, confident, utterly professional and hugely popular with his men and held in the highest regard by his seniors".
He said: "It was clear to me that he adored his job and his guardsmen in equal measure.
"He was tragically killed, on his 27th birthday, doing the job he loved and leading from the front.
"The gap he leaves in his company, the battalion and officers' mess is enormous. We all mourn his death and will miss his gentle yet keen sense of humour."
Operation Moshtarak is the biggest military offensive in Afghanistan since the Taliban were overthrown in 2001 and involves 4,000 British troops.
In total, more than 15,000 US, British and Afghan soldiers have embarked on a push to clear Taliban forces from Marjah and Nad Ali in Helmand.
A British soldier, from 1st Battalion Scots Guards, shot dead in the Nad Ali area also on Thursday has yet to be named. His family has been informed.
The first British soldier killed in the offensive was L/Sgt Dave Greenhalgh, 25, from Ilkeston, Derbyshire, who was serving with 1st Battalion Grenadier Guards.
He died in an explosion while on vehicle patrol near Lashkar Gah, in Helmand, on Saturday.
Defence Secretary Bob Ainsworth said he was "deeply saddened" to hear of the death of Lt Dalzell.
"He was clearly a gifted leader and an inspiration to his men, never asking them to do something he had not already done himself," he said.