Margaret Thatcher held secret talks with the king of Saudi Arabia over a £40bn arms deal, documents reveal.

The then-Prime Minister met King Fahd bin Abdulaziz Al Saud five months before the two governments publicly agreed a deal for the sale of arms to the Kingdom.

In 1985, government officials said the meeting was to discuss conflict resolution in the Middle East.

Newly released documents from the National Archives show the Foreign Office instead planned to use the meeting to "smoke out" the Saudis over the proposed Al-Yamamah deal.

The Al-Yamamah deal involved the Saudi government buying billions of pounds worth of Tornado jet fighters and parts from BAE Systems.

In a letter addressed to King Fahd, Thatcher said: "I was glad that we were able to discuss a further matter privately over lunch.

"I look forward to receiving your majesty's personal envoy soon, in order that we may conclude this matter successfully."

Five months after the meeting, defence ministers from both nations signed a memorandum of understanding in London for 72 Tornado jets, 30 Hawk training aircraft and a range of weapons, radar and spares, as well as a pilot-training programme.