ISLAMABAD - Pakistan's ambassador to the United States resigned on Tuesday, days after a Pakistani-American businessman said the envoy was behind a memo that accused the Pakistani military of plotting a coup in May.

Envoy Husain Haqqani said in a Twitter message that he had sent his resignation to the prime minister. State television said his resignation had been accepted.

"I have resigned to bring closure to this meaningless controversy threatening our fledgling democracy," he said in a statement released after his resignation.

"I have served Pakistan and Pakistani democracy to the best of my ability and will continue to do so."

Businessman Mansoor Ijaz, writing in a column in the Financial Times on October 10, said a senior Pakistani diplomat had asked that a memo be delivered to the Pentagon with a plea for U.S. help to stave off a military coup in the days after the May 2 U.S. raid that killed al Qaeda leader Osama bin Laden.

Ijaz later identified the diplomat as Haqqani.

No evidence has emerged that the military was plotting a coup and the Pentagon at the time dismissed the memo as not credible. Haqqani denies involvement in the memo. (http://r.reuters.com/wes25s)

"I still maintain that I did not conceive, write or distribute the memo," Haqqani told Reuters shortly after he resigned. "This is not about the memo," he continued. "This is about bigger things."

He declined to comment further.

Haqqani's resignation followed a meeting with Pakistan President Asif Zardari, the nation's powerful army chief General Ashfaq Kayani and its intelligence head, Lieutenant-General Ahmad Shuja Pasha.

A spokesman for the prime minister's office said Haqqani was asked to resign and there would be an investigation into the memo.

Haqqani is a former journalist who covered Afghanistan's civil war and later wrote a book on the role of radical Islam and the military in Pakistan.

With his crisp suits and colorful turns of phrase, he has developed close ties with Washington's top power brokers as Pakistan's envoy since 2008.

In the past year, he has sought to ease tempers in both capitals and find common ground during an extraordinarily tense period in U.S.-Pakistani relations that included the bin Laden raid, the jailing of a CIA contractor, and U.S. accusations that Pakistan backed a militant attack on the U.S. Embassy in Kabul.

He is close to Zardari but estranged from Pakistan's military.

Tensions between Pakistan's civilian government and military have bedeviled the nuclear-armed South Asian country for almost its entire existence, with the military ruling the country for more than half of its 64-year history after a series of coups.

Haqqani's resignation was seen by many analysts as further weakening the civilian government, which is already beset by allegations of corruption and incompetence.

"They (the military) may expect much more from the government, much more beyond the resignation of Husain Haqqani, because they see that everybody perceived to be involved in this affair will be seen as anti-military and by implication anti-state," said Imtiaz Gul, a security analyst in Islamabad.

IMPACT ON U.S.-PAKISTAN TIES?

Haqqani's successor might include a diplomat with a less complicated relationship with the military, perhaps Pakistani Foreign Secretary Salman Bashir or Pakistan's envoy to the United Nations, Hussain Haroon.

"Whether Pakistan's people or its military will be represented in DC will become evident when Husain Haqqani's replacement is announced," Ali Dayan Hasan, representative for Human Rights Watch in Pakistan, said on Twitter.

Vali Nasr, a former senior State Department official who worked on Pakistan, said the crux of the affair was not Haqqani's role but whether Zardari would come to be seen as having directed the memo, which would imply the president had gone outside Pakistan to request urgent assistance against his own military.

"At what point would the issue escalate to Haqqani was acting on Zardari's behest? That would really create massive tension between the military and Zardari."

Nasr said the issue would be unlikely to have a major impact on the strained U.S.-Pakistan relationship unless it seriously weakened or toppled the civilian government.

As U.S. officials focus on thorny diplomatic and security issues with Pakistan, Haqqani's departure did not immediately make many visible ripples in Washington. The State Department said it had not been notified of Haqqani's departure and the Pentagon declined comment.

Democratic Senator John Kerry, who has been heavily involved in U.S.-Pakistani relations, said he was sorry to learn of Haqqani's resignation.

Kerry said he respected the Pakistani government's decision but that Haqqani would be missed "as we continue to work through the ups and downs of our relationship."

Ijaz said initially he believed Haqqani was acting under the authority of Zardari, but said later he was not sure how involved Zardari was in the affair.

Mark Siegel, a lobbyist who represents the Pakistani government in Washington, said Zardari called him when the Financial Times story appeared, asking his law firm to initiate libel proceedings against the newspaper and Ijaz.

Siegel advised Zardari against filing a case because he judged it difficult for a public figure to win a libel case in a U.S. court.

"He was irate and said the memo was a total fabrication," Siegel said. Siegel, who has known Zardari for 25 years, said he was absolutely certain that Zardari had known nothing about the memo.