BORIS Johnson wrote to Cabinet colleague Liam Fox to reassure him over arms exports to Saudi Arabia, official documents have shown.

The Foreign Secretary’s letter came after the International Trade Secretary had deferred decisions on export licences for equipment destined for the Saudi air force which could have been used in Yemen.

The letter was sent on November 8, a month after an air strike by the Saudi-led coalition killed more than 140 people attending a funeral in Yemen.

Correspondence between the ministers, which was reported by the Guardian, revealed that Fox acknowledged that the risks of continuing arms exports to Saudi Arabia was “extremely finely balanced” in part because of the “grave situation” in Yemen.

In the letter, Johnson wrote: “I am aware you have deferred a decision on four export licence applications to supply the Royal Saudi Air Force with equipment which could be used in the conflict in Yemen.”

He said the commitments given by the Saudis after the October 8 air strike meant that the threshold for refusing a licence – the assessment that there is a “clear risk” that the equipment could be used in violation of international humanitarian law – had not been reached.

In the letter, Johnson told his colleague: “The issue is extremely finely balanced, but I judge at present the Saudis appear committed to taking action to address failures/individual incidents.”

The Guardian reported that Fox subsequently recommended that the licences be approved, but in his reply on November 17 the International Trade Secretary acknowledged there were risks to the decision. He wrote: “I agree that this is a complex situation and the issue of clear risk is extremely finely balanced.”

Agreeing to continue assessing export licences on a case-by-case basis, he added: “In doing so I want to be very clear with you about the risks inherent in making this decision, not just because of the grave situation in Yemen.”

Fox also insisted that the Foreign Office should provide regular updates to his department.

The letters were disclosed as part of High Court proceedings over the continued sale of arms to Saudi Arabia.

The Campaign Against Arms Trade has accused the Government of unlawfully failing to suspend the sale of UK arms to Saudi Arabia, despite evidence that the Gulf state is guilty of “repeated and serious breaches” of international humanitarian law.

The campaign group have argued that UK fighter jets and bombs sent to the desert kingdom have been used in the conflict in Yemen in which thousands have died.

A Government spokesman said: “The UK is playing a leading role in work to find a political solution to the conflict in Yemen and to address the humanitarian crisis.

“We operate one of the most robust export control regimes in the world and keep our defence exports to Saudi Arabia under careful review.

“Given the current legal proceedings we will not be commenting further outside of court at this stage.”

Labour’s Shadow Foreign Secretary Emily Thornberry said: “It should not be left to the courts to rule whether the export licences for these arms sales should have been granted.

“It should be for this Government to show some long-overdue caution and concern about the way the Saudi campaign is being conducted, the devastating humanitarian crisis that campaign is helping to cause, and the blatant failure to ensure any proper, independent investigation of these alleged crimes.”