Oxfam and Amnesty International UK have enlisted the help of the second largest trade union Amicus to give extra impact to its campaign on tighter arms controls.
The three organisations jointly signed a letter that appeared in the Financial Times last week calling on the Government to close a loophole in its new controls on arms exports. It is feared brokers will be able to avoid most controls by conducting their deals abroad.
"By working with Amicus we're showing the Government that this is a broader coalition than just the usual suspects," said Oxfam spokesman Brendan Cox.
The organisations plan to take a more international approach to arms trade campaigns, which is why they chose to contact a newspaper with a global readership.
Amicus has never previously collaborated with the voluntary sector. Spokesman Richard O'Brien said: "As a left-of-centre organisation, we have sympathy with much of Amnesty and Oxfam's agenda."