"It was his fifth trip to Afghanistan, and he had forged friendships with a number of the soldiers serving out there," she said on the paper's website.
The UK's foreign office said Hamer was the first British journalist to die in Afghanistan.
Gordon Brown, the UK prime minister, said in a statement: "My heartfelt thoughts and sympathies are with the families, friends and colleagues of Rupert and Philip.
"Their courage, skill and dedication to reporting from the frontline was incredibly important and ensured that the world could see and read about our heroic troops."
Michelle Lang, a Canadian reporter of the Calgary Herald newspaper, was killed alongside four soldiers in Kandahar province on December 30 when a roadside bomb exploded beneath their armoured vehicle.
The Taliban claimed responsibility for that attack.
A total of 1,575 troops in the military coalition have died in Afghanistan since 2001, according to the icasualties.org website.
There has been an upsurge of violence in recent months.
Three journalists, including Lang, died in Afghanistan last year, according to a tally by the International News Safety Institute.
Reacting to Hamer's death, Bob Ainsworth, the UK's defence secretary, said: "The sacrifice of service personnel is well documented and rightly respected, but this news demonstrates the risks also faced by journalists who keep the public informed of events on the front line."