A UK-based NGO, Campaign Against Arms Trade (CAAT), has brought the legal action, accusing the government of contributing to breaches of international law and the world's largest humanitarian disaster, which has claimed tens of thousands of lives.

The judicial review is expected to last until the end of the week.

The NGO brought the legal challenge after Britain announced in summer 2020 that it was resuming arms sales to Saudi Arabia.

Ahead of the hearing, CAAT's media coordinator Emily Apple accused London of being a "government that cares more about profit than war crimes and the deaths of civilians".

The NGO initially won its case against the government in 2019, when the Court of Appeal ruled that the UK's licensing of arms sales was unlawful.

It said the government had failed to assess properly whether the arms sales violated its commitments to human rights and ordered it to "reconsider the matter".

While serving as international trade minister, Liz Truss then conducted a review and announced in 2020 that export licences would restart.

She insisted Riyadh "has a genuine intent and the capacity to comply with IHL (international humanitarian law)", despite "isolated incidents".

CAAT accused Truss of "paying lip service" to the need to review sales.

Its spokeswoman condemned Truss's reference to "isolated incidents" as "total nonsense and deeply offensive to all the Yemeni people who've had their lives destroyed by UK weapons".

CAAT said the UK government has licensed sales to Riyadh of weaponry including combat aircraft, guided bombs and missiles, with a published value since 2015 of pound7.9 billion ($9.8 billion).

It said the UK is one of the leading suppliers of arms to Saudi Arabia, along with the United States.

Martin Butcher, peace and conflict advisor at the charity Oxfam, said Saudi airstrikes "are responsible for a larger proportion of the attacks" on civilians in Yemen.

"It's essential that the legality of UK arms sales is examined and arms sales must be immediately stopped," he said.

In 2021 charities criticised the British government for slashing in half its humanitarian aid to war-torn Yemen.

The UK government says Britain is the world's second largest defence exporter behind the United States. The sector had a turnover of pound25.3 billion in 2020.