The UK needs to tighten arms controls to prevent weapons being secretively traded to war-torn areas, human rights group Amnesty International says.
Amnesty called for tougher laws to control the arms trade
It claims there is a network of arms transporters and brokers - including UK companies - "fuelling the world's bloodiest conflicts".
Its report said hundreds of thousands of weapons and millions of rounds of ammunition are transported yearly.
Members will be marching on Parliament to lobby MPs for tougher controls.
"Weak and outdated arms controls are failing to stop brokers and transporters from fuelling massive human rights abuse around the world," Amnesty said.
The group's International UK director Kate Allen said: "The arms trade is out of control and costing hundreds of thousands of lives every year.
"The UK has a real chance to do something about it when the UN meets in June - and we want MPs to tell the government that their constituents really care about this issue."
The report, by Amnesty and TransArms, said there were arms brokers and transporters in the UK, Italy, the Netherlands, Switzerland, Balkans, China, Israel, Ukraine, United Arab Emirates and the US.
They deliver arms around the world, "with an ever-greater proportion going to developing countries where they have fed some of the most brutal conflicts".
Last year thousands of weapons and rounds were transported "supposedly to Iraq, from Bosnia and Herzegovina's war-time stockpiles," the report alleged.
Ms Allen said: "Arms brokers and transporters have helped deliver the weapons used to kill and rape civilians in Sudan and the Democratic Republic of Congo.
"Yet only 35 states have laws to regulate brokers."
She called for an "arms trade treaty to bring the whole industry under control, and stop weapons being sold to human rights abusers".
The group wants immediate establishment of national laws to prevent arms dealings that contribute to gross human rights violations.
It also called for states to make the violation of UN arms embargoes a crime.