David Cameron's recent trip to the Middle East was "morally obscene", Green MP Caroline Lucas has claimed.
The Green Party leader said Mr Cameron had travelled to the region accompanied by a "delegation of arms traders".
Nearly a third of businessmen accompanying the PM on his trip to Egypt, Kuwait, Qatar and Oman were from the defence or aerospace industry.
The government defended the trade mission and said the UK has among the tightest arms sales rules in the world.
The prime minister became the first world leader to visit Egypt since its former President Hosni Mubarak was forced out of office two weeks ago.
He visited Tahrir Square, the focus of the anti-Mubarak protests, and met figures from the pro-democracy movement.
Talking about recent events in the the region at her party's spring conference, Ms Lucas described the sense of awe she had felt on seeing "hundreds and thousands risk their lives for democracy and the rule of law".
She said that upon first seeing Mr Cameron in Egypt she believed he was there to "express solidarity with the pro-democracy movement".
But she said her view changed when she realised that senior executives from defence companies were amongst those participating in the visit.
She talked of the "horrifying reality that [David Cameron] was there, in the Middle East, at a time of such violence and unrest with a delegation of arms traders to sell more arms".
"The blatant opportunism of this visit is morally obscene," she told party activists.
In a later interview with the BBC, the Brighton Pavilion MP called the insensitivity of the visit "eye watering" and argued for a return to an "ethical foreign policy".
Labour also expressed concerns about the timing of the trade mission coming as it did amid violent crackdowns against anti-government protests in Bahrain and Libya.
The Foreign Office revoked a series of export licenses to Bahrain and Libya, covering tear gas and gun components, following violence in both countries.
During the trip, Mr Cameron defended the make-up of the delegation, saying it was legitimate for British firms to be doing business with allies, that the UK had tight rules on arms sales and a properly regulated defence industry was perfectly right and proper.
"Democracies have a right to defend themselves," he said on Tuesday.
"The idea that Kuwait should not be able to have its own armed forces, that it is unable to defend its own country and take part in defence trade, is an extraordinary argument."