Publishing a report for its Control Arms Campaign on Wednesday, international charity Oxfam said the UK was supplying weapons to countries where human rights abusers could use them to kill, torture and rape at gunpoint.
Director of policy Justin Forsyth accused the government of exercising "double standards on export licences - making it easier to obtain a licence for weapons components while tightening up on licences for entire weapons.
"The government has put lives at risk by setting up false and dangerous double standards. Whether a machine gun comes in pieces or ready-made, the suffering it can cause in the wrong hands is just the same.
"These aren't simply nuts and bolts we're selling. These components include firing mechanisms, bomb-making equipment, guidance systems and gun barrels."
The government rejected the report called Lock, Stock and Barrel.
Foreign Office minister Baroness Symons said the report had no evidence to support its claim Britain was too lax on exports of arms components.
"It is simply not the case. We take full account of what the components might be used for ... we simply would not issue a licence where there was an unacceptable risk of it being misused or diverted."
The report said there had been an eleven-fold increase since 1998 in the number of different arms components licensed for export to countries including Zimbabwe, Israel, Indonesia, Colombia, Nepal and the Philippines.
"These aren't simply nuts and bolts we're selling. These components include firing mechanisms, bomb-making equipment, guidance systems and gun barrels"
Oxfam's director of policy
"Arms components are clearly central to the operation and maintenance of weapons systems, and if it is wrong to export a whole system, then it is equally wrong to licence the parts that make that system work," the report said.
It said the export of weapons parts rather than entire weapons systems created "a smokescreen that hides the true extent of the British arms trade".
Other rights groups
Lesley Warner, spokeswoman for Amnesty International UK, also said components for deadly weapons were being sold to known human rights abusers.
"It doesn't take much to re-assemble them. And from there it takes even less to kill, to torture or to rape at gunpoint."
Foreign Secretary Jack Straw provoked fury among members of the ruling Labour party when he announced new export guidelines in July 2002 which allowed the sale of parts of military equipment to countries such as Israel.
Rebecca Peters, of the International Action Network on Small Arms, said these new criteria "allow arms components to go to a whole host of countries where human rights abuses are common.
"It seems that the government is attending to the needs of the British defence industry above the human rights of people living in countries where the weapons will be used."