Saudi Arabia's King Abdullah has accused Britain of not doing enough to fight international terrorism, which he says could take 20 or 30 years to beat.
He was speaking in a BBC interview ahead of a state visit to the UK - the first by a Saudi monarch for 20 years.
He also said Britain failed to act on information passed by the Saudis which might have averted terrorist attacks.
BBC security correspondent Frank Gardner says Whitehall officials have strenuously denied this.
King Abdullah is expected to arrive in the UK on Monday afternoon; his visit begins formally on Tuesday.
In the BBC interview he said the fight against terrorism needed much more effort by countries such as Britain and that al-Qaeda continued to be a big problem for his country.
BBC world affairs correspondent John Simpson says King Abdullah is annoyed that the rest of the world has largely failed to act on his proposal for a UN clearing house for information about terrorism.
Speaking through an interpreter, the Saudi monarch said he believed most countries were not taking the issue seriously, "including, unfortunately, Great Britain".
"We have sent information to Great Britain before the terrorist attacks in Britain but unfortunately no action was taken. And it may have been able to maybe avert the tragedy."
The Saudi leadership maintains that it passed the UK information that might have averted the London bombings of 2005 if it had been acted on.
An investigation by Parliament's Intelligence and Security Committee (ISC) found no evidence of any intelligence passed on by the Saudis that could have prevented the 7 July 2005 bombings, the BBC's Frank Gardner said.
The king's visit has provoked controversy over Britain's relationship with Saudi Arabia.
A demonstration is planned outside the Saudi embassy in London later in the week in protest at the country's human rights record.
And acting Liberal Democrat leader Vince Cable has announced he is boycotting the visit, citing the corruption scandal over Al Yamamah arms deal, and the Saudis' human rights record.
"I think it's quite wrong that as a country we should give the leader of Saudi Arabia this honour," he said.
Kate Allen, director of Amnesty International UK, urged Prime Minister Gordon Brown to tell the Saudis that their human rights record was "totally unacceptable".
She added: "Mr Brown's message should be - reforms need to come, and they need to come quickly."
The Foreign Secretary, David Miliband, has pulled out of a scheduled meeting with the Saudi delegation to spend time with his wife and their newly-adopted second son.
It is understood that Mr Miliband is returning from the United States, where he was present at the birth of Jacob. He was replaced by Foreign Office Minister Kim Howells.
A spokeswoman for the Foreign Office said that the decision to invite King Abdullah was a reflection of the "long-standing friendship" between the two nations.