London's High Court has ruled that Britain's multibillion-pound arms sales to its ally Saudi Arabia is lawful.

The Campaign Against the Arms Trade (CAAT) had sought an order to block the export licences for British-made bombs, fighter jets and other munitions.

The campaigners argued they were being used by the Saudi-led Arab coalition in Yemen in violation of international humanitarian law.

But on Monday, the High Court said "the claimant's claim for judicial review is dismissed".

The court ruled that there had been extensive political and military engagement with Saudi Arabia regarding the conduct of operations in Yemen and the Saudis had "sought positively to address concerns about international humanitarian law".

Some of the evidence in the case was presented in secret on national security grounds.

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CAAT said it would appeal against the decision, calling the ruling "very disappointing".

"If this verdict is upheld then it will be seen as a green light for government to continue arming and supporting brutal dictatorships and human rights abusers like Saudi Arabia that have shown a blatant disregard for international humanitarian law," CAAT's Andrew Smith said in a statement.

An annual report by UN experts who monitor sanctions and the conflict in Yemen, seen by Reuters news agency in January, said the Saudi-led coalition had carried out attacks in Yemen that "may amount to war crimes", accusations that Riyadh has rejected.

CAAT had been seeking a judicial review over the government's decision to allow arms exports to continue to Saudi Arabia, a major customer for British defence companies and an important British ally in countering "terrorism".

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Al Jazeera's Paul Brennan, reporting from London, said the judges involved said "their job was not to look at the exports themselves, but ... at the decision-making process".

The court said that "Saudi Arabia has been, and remains, genuinely committed to compliance with International humanitarian law; and there was no 'real risk' that there might be 'serious violations' of International humanitarian law [in its various manifestations] such that UK arms sales to Saudi Arabia should be suspended or cancelled".

In 2015, Saudi Arabia and its allies launched an air campaign aimed at reversing Houthi military gains and backing Yemen's UN-recognised government.

Since then, the UK has licensed more than $4.2bn worth of arms exports to Saudi Arabia. In addition, the government has signed off another half a million dollars for armoured vehicles and tanks.

Two years of conflict have killed more than 10,000 people, wounded tens of thousands and displaced millions.

Amnesty International said the ruling was a "deadly blow" to Yemeni civilians.

"This is a deeply disappointing outcome which gives a green light to the UK authorities - and potentially Saudi Arabia's other arms suppliers - to continue authorising arms transfers to the Kingdom despite the clear risk they will be used to commit violations," James Lynch, Amnesty's head of arms control and human rights, said in a statement.

George Graham of Save the Children said there was clear evidence that the Saudi-led coalition is "killing children in repeated violations of international humanitarian law."

"This is not a point of contention - the evidence is overwhelming. It has been documented by UN reports, by aid groups on the ground and by credible human rights organisations," Graham said. "We can be proud of the difference the UK is making in Yemen - British aid is keeping children alive," he said.

"But at the same time, our weapons are helping to fuel a war that is killing children and destroying their schools and hospitals."