The state visit to Britain by King Abdullah of Saudi Arabia was last night plunged into controversy when Vince Cable, acting leader of the Liberal Democrats, declared he would boycott the event which begins today.

It is highly unusual for a leader of an opposition party to snub a royal visit. Foreign Office officials could not immediately recall any precedent. A departmental spokeswoman noted: "Vince Cable's decision to decline the state banquet is a matter for him."

The MP for Twickenham, who is caretaker leader of the LibDems following the resignation of Sir Menzies Campbell, cited his reasons for the boycott as the corruption scandal over the infamous al Yamamah arms deal and the Middle East state's record on human rights - particularly its treatment of women - as well as the conduct of some members of the Saudi royal family.

Yesterday in Riyadh, a man was publicly beheaded with a sword after being convicted of a fatal shooting. The execution brings to 117 the number of people beheaded this year in the Islamic kingdom, three times the number in 2006.

Human-rights campaigners point to how Saudi women still have to carry a yellow card with their male guardian's permission to travel and to how critics of the regime are regularly imprisoned.

In a letter to the Saudi embassy in London, Mr Cable said: "I have introduced three debates in parliament this year expressing serious concerns over the al Yamamah contract and the corruption allegedly involved.

"I have also been critical of members of the Saudi royal family and the Saudi record on human rights, including its maltreatment of British citizens. In my opinion, it is quite wrong for the British government to have proposed a state visit at this time. Therefore, it would, I believe, be inappropriate for me to participate in a ceremonial state visit against this background."

Mr Cable's decision to boycott the visit follows the controversy which erupted last year when Tony Blair halted a long-running Serious Fraud Office inquiry into the £40bn al Yamamah deal signed by Margaret Thatcher in the 1980s. The then prime minister argued that Saudi security co-operation in the fight against international terrorism could be jeopardised if the investigation continued.

However, critics claimed that Mr Blair was more concerned about how Britain could lose out on a fresh £20bn contract to supply the Saudis with 72 Eurofighters.

The visit of King Abdullah will also see a public protest planned for Wednesday when the king is due to meet Gordon Brown in Downing Street. Labour left-winger John McDonnell said that protesters would be staging a demonstration outside the Saudi embassy in Mayfair.

"The British people will be aghast at the government entertaining on a state visit one of the most prominent anti-democratic and human rights abusing leaders in the world," said the London MP.

"Why is it that in the same breath the Prime Minister condemns the lack of democracy in Burma and the abuse of human rights in Zimbabwe but remains silent when it comes to the Saudi dictatorship?

"Why is it the Prime Minister speaks about democracy and civil liberties one week and the next week is willing to sit down to dinner with King Abdullah, who refuses to allow any political opposition and any basic civil liberties within the country he rules?"

The Foreign Office spokeswoman said that the decision to invite King Abdullah now reflected the "long-standing friendship" between the two nations. She added that British and Saudi interests were "intertwined and inseparable" across a range of issues from counter-terrorism to ensuring stability in the Middle East.

King Abdullah, who arrives today, will be officially welcomed tomorrow by the Queen at a short ceremony on Horse Guards Parade. The highlight of the four-day trip will be a state banquet hosted by the Queen.

Today, David Miliband, the Foreign Secretary, will hold the third meeting of the UK-Saudi "Two Kingdoms Dialogue" in Lancaster House.