David Cameron has said he is "deeply saddened" to hear of the death of King Abdullah of Saudi Arabia.
The Prime Minister, who visited Saudi Arabia in 2012, said:
He will be remembered for his long years of service to the Kingdom, for his commitment to peace and for strengthening understanding between faiths.
My thoughts and prayers are with the Saudi Royal Family and the people of the Kingdom at this sad time.
I sincerely hope that the long and deep ties between our two Kingdoms will continue and that we can continue to work together to strengthen peace and prosperity in the world.
Politicians have been hailing the King a 'reformer', but rights groups said his record was dire.
At the age of 88, Her Majesty is now the oldest monarch after the death of Saudi Arabia's 90-year-old King Abdullah.
King Abdullah set himself apart from the rest of the Saudi royal family for his love of the simple life and his embrace of cautious reform.