Lance Corporal George Solomou, 38, who handed in a letter of resignation to his commanding officer at a Territorial Army base in south London, said a number of other reservists felt the same way about the conflict.
"There is a sizeable minority within the Territorial Army that do not agree with this war, but many soldiers do not realise they are legally entitled to conscientiously object to serving in a war," he told reporters in London on Tuesday.
"Soldiers have a moral, public and political duty to defend democracy. When democracy fails as it did when (Prime Minister Tony) Blair led us into this illegal war, they have a duty to uphold democracy by not taking part in it."
Solomou had been in the Territorial Army for five years as a member of the Royal Army Medical Corps. He did not serve in Iraq.
The US-led, British-backed war to remove Saddam Hussein in March 2003 had been "illegal and immoral", he said.
In the letter, Solomou said foreign troops should get out of Iraq immediately.
"I am resigning as a conscientious objector because I don't want any part of it and also because I hope my action might just encourage other soldiers to speak out or opt out"
"The continuing occupation is a disaster for the people of Iraq and a nightmare for the British and US troops on the front line," he said.
"I am resigning as a conscientious objector because I don't want any part of it and also because I hope my action might just encourage other soldiers to speak out or opt out."
Britain currently has around 8,960 military personnel serving in Iraq, according to the Ministry of Defence.
A "relatively small proportion" of these are reservists, but exact figures are not kept, a ministry spokesman said.