THERESA May has been accused by the SNP of leading an “increasingly morally defunct government” as the row over her visit to Saudi Arabia showed no signs of abating.

The Prime Minister insisted it was in the "national interest" to maintain good relations with the Gulf state despite its controversial record on human rights as she flew into Riyadh on the second leg of a short Middle East visit to boost trade and security ties.

Mrs May symbolically chose to go headscarf-free during the two-day trip to the kingdom despite convention dictating that women cover their hair.

"The May doctrine of foreign policy is that everything we do is in our British national interest," declared the PM.

"It is in our British national interest to have good relations around the world so we can trade around the world. That brings jobs and prosperity to the UK.

"It's also in our national interest to ensure that we are working with others around the world to maintain our safety and security.

"And yes, it is in our national interest to ensure that the values that underpin us as Britons are values that we promote around the world and that's what we do," she insisted.

It is the first time Mrs May has visited as Prime Minister and she expressed her hope was to inspire oppressed women in the state, whose actions are severely restricted under the system of so-called male guardianship.

As Home Secretary, she held talks in the country without covering up her head but she has previously worn a headscarf during a visit to a mosque in Pakistan.

Mrs May strongly denied Britain was selling its principles for trade deals despite widespread criticism over UK arms deals with the kingdom.

The PM has faced repeated calls to suspend the sales following claims of human rights abuses in Yemen under the Saudi-led coalition bombing campaign.

Labour called on Mrs May to halt arms exports to the state immediately and urged her to back an independent investigation into alleged war crimes in the conflict.

Jeremy Corbyn, the party leader, said unless she challenged the Saudi Government over what he termed were “its abuses,” then it would be “clear she is ready to sacrifice human rights and security on the altar of the arms trade”.

However, the PM insisted the UK must engage with the Saudis rather than snipe from the sidelines.

"The important thing for the United Kingdom when we meet people and we want to raise issues of human rights - and that may be in a number of countries around the world - is if we have the relationship with them, then we are able to do that.

"So, rather than just standing on the sidelines and sniping, it's important to engage, to talk to people, to talk about our interests and to raise, yes, difficult issues when we feel it's necessary to do so."

Asked if the UK was selling its principles cheaply for the sake of trade and arms deals, she replied: "No, we are not doing that. What we are doing is continuing the links that we have had for a long time with countries that are important to us around the world."

But the SNP’s Tasmina Ahmed-Sheikh accused Mrs May of double standards.

“The Prime Minister’s comments highlight the direction this increasingly morally defunct UK Government is intent on steering this country towards, regardless of who it is shaking hands with or what they are selling; all in the name of shared interests.

“It is beyond words that she is proud of a relationship that sees the UK selling arms and military equipment with one hand and handing aid to its victims with the other.”

The Saudis back the war-torn country's internationally recognised government against Iran-backed Houthi rebels.

Britain has continued to allow arms sales, with more than £3.3 billion of exports since the bombing began in March 2015.

At least 10,000 people have been killed during the war, according to the United Nations.