Four of those on board, three soldiers and an airman, were stationed at RAF Odiham in Hampshire. The fifth, an Army reservist, was based in London.
The Ministry of Defence said the crash near Kandahar air base appeared to have been a "tragic accident".
It represents the third single biggest loss of life of British troops in Afghanistan since the conflict began.
The families of all five servicemen have been told.
Three of the soldiers on board the helicopter were from the Army Air Corps, based at RAF Odiham in Hampshire, the MoD said late on Saturday.
A fourth member of the crew was serving with the Royal Air Force and was also based at Odiham, while the fifth man was an Army reservist from 3 Military Intelligence Battalion, in London.
Maj Gen Richard Felton, Commander Joint Helicopter Command, said: "It is with great sadness that we must confirm that five UK service personnel have been killed in this incident which, at this early stage, would appear to have been a tragic accident.
"Events like this, whilst mercifully rare, remind us of the risks our personnel face in their work in Afghanistan as we approach the conclusion of the combat mission later this year.
"Our thoughts are with the families and loved ones of those who have lost their lives."
Prime Minister David Cameron said his "heart goes out to the families and friends of those killed in this terrible tragedy".
Labour leader Ed Miliband said it was "tragic and poignant reminder of the sacrifices made by our armed forces in serving our country with bravery and distinction".
It is the first fatal accident involving a UK military helicopter in Afghanistan since the conflict began in 2001.
BBC defence correspondent Caroline Wyatt said sources had suggested the cause of the crash may have been "technical problems" on board the helicopter, despite claims by the Taliban that its fighters had shot it down.
She said the location of the crash - close to the border with Pakistan - has also led to speculation that the helicopter could have been taking part in a special forces mission.
It is understood the helicopter was a Westland Lynx Mk 9.
A Ministry of Defence spokesman said: "We can confirm that a UK helicopter crashed in southern Afghanistan today.
"The incident is under investigation and it would be inappropriate to comment further until families have been notified."
The crash is understood to have happened around 30 miles from the Pakistan border, near Kandahar air base in the Takhta Pul district.
The deaths bring the number of British forces killed in the conflict in Afghanistan to 453.
The fatal crash comes after a Nimrod surveillance aircraft exploded in mid-air while supporting Nato ground operations near Kandahar, killing all 14 servicemen on board, in September 2006.
This incident remains the biggest single loss of UK life at one time in Afghanistan.
In 2012, six British soldiers were killed when a Warrior armoured fighting vehicle in Kandahar province was hit by an explosion.
The International Security Assistance Force (ISAF) coalition of international forces said it was still in the process of reviewing the circumstances of Saturday's helicopter crash.
"Our thoughts and prayers go out to the family and friends affected by this tragic event," it said in a statement.
Defence analyst Paul Beaver said all the indications are that the crash involved a Lynx helicopter, which he said has an "exceptional record".
He said the light utility helicopter is likely to have been operating in "fairly mountainous areas", or in "challenging" terrain.
"The key now will be looking at the weather," Mr Beaver told the BBC.
"If enemy action is not suspected, which is what the MoD is saying, then you have to look at whether this might well be weather-related."
The crash brings the total number of international troops killed in Afghanistan in April to seven.
It comes after the last major helicopter crash took place in December last year, when seven Americans and four Afghans died.
In August 2011, the Taliban shot down an American Chinook near Kabul, killing 30 Americans and eight Afghans in the deadliest single incident for US troops since the war began.
Concerned family members can contact the MoD's Joint Casualty and Compassionate Centre on 08457 800 900.
Five British service personnel have been killed in a UK helicopter crash in southern Afghanistan.