Britain's ambassador to Washington has been criticised by human rights campaigners after he called on the US to use British-made missiles on its Reaper drones.

Peter Westmacott, the UK's senior diplomat to the US, made the call in an article in which he said that the US military had just finished testing a British-designed missile, the dual-mode Brimstone, at a test facility in California.

"The tests successfully proved that the Brimstone is compatible with one of America's main unmanned aerial vehicles, the Reaper, and is a more accurate weapon than anything comparable on the market," wrote Westmacott in the piece for Defense One, a US defence site.

"It makes sense to buy equipment that your allies have already developed, rather than invest millions or billions of dollars of taxpayers' money to duplicate what's already available. This more cost-effective approach to procurement is especially important when budgets are tight. I'm tempted to say it's not rocket science – but in this case that's not strictly true!"

Reprieve, a London-based human rights group, wrote to the foreign secretary on Tuesday to ask what the position of the UK government was on the legality of US drone strikes in Yemen and Pakistan and what assessment it had made of the potential legal and diplomatic consequences of supplying UK-built missiles for use in strikes by the US.

Drawing attention to Westmacott's recent comments, which it described as enthusiastic salesmanship, Reprieve said: "As you will be aware, the Reaper – along with its smaller cousin, the Predator is best known for its use by the CIA and other secretive US agencies in carrying out covert strikes in Yemen and Pakistan."

"Such strikes are in violation of international law, and known to have killed large numbers of civilians outside of declared war zones."

Reprieve claimed that the potential future supply of such missiles was "an alarming prospect when one considers that, as recently as December, one such strike was reported as having 'turn[ed] a wedding into a funeral'."