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"The Prime Minister looks forward to welcoming the Crown Prince to the UK in the New Year," a spokeswoman said following a call between May, the crown prince and King Salman bin Abdulaziz.

May visited Riyadh in November, where she met the the 32-year-old defence minister and held talks on issues including Qatar, where Saudi Arabia is leading a blockade in the emirate accusing it of supporting Islamist extremists and of being too close to Iran, Riyadh's arch-rival.

Doha denies the allegations and instead accused the Saudi-led bloc, which includes Bahrain, the United Arab Emirates and Egypt, of aiming to incite regime change in Doha.

The British leader also discussed the Yemen war with MbS, who is the architect of the kingdom's war against Houthi rebels. 

London has signed off on more than £3.3 billion (€3.7 billion, $4.4 billion) worth of arms sales to Riyadh since March 2015. 

During that time Saudi Arabia has embarked on a bombing campaign in Yemen that has been condemned for contributing to a humanitarian disaster and allegedly violating international humanitarian law. More than 10,000 people have since been killed.

In September, a War Child report found that since the Saudi-led coalition began its intervention in Yemen in 2015, British weapons companies including BAE systems and Raytheon have earned revenues of more than $8 billion from dealings with Saudi Arabia, generating profits estimated at almost $775 million.

Humanitarian aid to Yemen eclipses the tax income generated from the weapons sold that are fuelling the crisis, the report also found, with weapons sales to Saudi Arabia resulting in approximately $13 million in corporation tax in 2016, yet during 2017, the UK will spend $188 million in humanitarian aid to Yemen.

The UK frequently justifies its close relationship with Saudi Arabia by highlighting how it delivers essential counter terrorism intelligence.

Agencies contributed to this report