The British government's determination to continue selling weapons to Saudi Arabia – despite credible reports that the Saudis are committing war crimes in Yemen – is facing its most serious challenge yet. A parliamentary committee – the Committee on Arms Export Controls (CAEC) – is due to report later this week and is expected to accuse the government of flouting its own rules on arms sales.

A draft version of the report, leaked to the BBC's Newsnight programme, says:

"The weight of evidence of violations of international humanitarian law by the Saudi-led coalition in Yemen is now so great that it is very difficult to continue to support Saudi Arabia while maintaining the credibility of our arms licensing regime."

Considering the scale and history of British arms sales to Saudi Arabia, the report says it "seems inevitable" that any violations of international law by the Saudi-led coalition in Yemen would involve arms supplied by the UK. It adds that the fact the government has not supported efforts to establish an independent international inquiry has allowed for the transfer of items to Saudi Arabia "very possibly" in contravention of the UK's legal obligations:

"While such doubt and uncertainty about IHL (international humanitarian law) compliance in Yemen exists, the default position of the UK government should not be to continue to sell weapons."

The government has a legal obligation not to grant arms export licences where there is "a clear risk" that items "might be used" in committing serious violations of international humanitarian law. The report continues:

"We have found that the government's arms export licensing regime, which it repeatedly asserts is robust, is in fact to a large extent opaque, and the government too often relies on assertion rather than positive evidence."

Members of the Committee on Arms Export Controls are drawn from four other committees with an interest in arms sales: defence, foreign policy, international development and business. Half its 16 members belong to the ruling Conservative party but the government's policy on arms sales to Saudi Arabia has been criticised by some Conservative MPs as well as opposition MPs. The committee has no power to change policy but it can summon ministers for questioning and the government may find its views difficult to ignore.

The Saudis are clearly taking these developments seriously: foreign minister Adel al-Jubeir has flown over to London and will be giving a private briefing to some MPs later today.

It remains to be seen whether Jubeir will succeed in changing the committee's view. The fact that a draft version of its report was leaked to the BBC means that any softening of text in the final version is now certain to be noticed.

In an embarrassing move last July, the government was forced to retract a serious of misleading statements made in parliament in which ministers claimed the Saudis had not broken international humanitarian law in Yemen. MPs had repeatedly been told: "We have assessed that there has not been a breach of IHL." In the "correction" issued in July, this was changed to say: "We have not assessed that there has been a breach of IHL". In other words, the government is no longer actively denying war crimes in Yemen but saying merely that it is not aware of any.

In attempting to maintain its latest position, the government basically has to close its eyes to growing evidence from human rights organisations and others that the Saudis are indeed committing war crimes in Yemen.

One recent example was the airstrike last month on a hospital supported by Médecins Sans Frontières in which 14 people, including an MSF staff member, were killed. It was the fourth time in 12 months that MSF facilities in Yemen had been hit, even though MSF says it keeps the Saudis informed about its activities and provides coordinates of the places where it is working.

Meanwhile, foreign secretary Boris Johnson continues to exonerate the Saudis. In a written statement to parliament, he said: "The key test for our continued arms exports to Saudi Arabia in relation to international humanitarian law is whether those weapons might be used in a commission of a serious breach of international humanitarian law. Having regard to all the information available to us, we assess this test has not been met."