America's continuing imprisonment of a British Guantanamo Bay inmate is "insulting" and "doing increasing damage to the Anglo-American relationship," a senior British politician has said.

The unusually frank comments — made by ruling Conservative Party MP Andrew Mitchell to VICE News — highlight the UK's increasing frustration over the case of Shaker Aamer, a Saudi Arabia citizen and British resident who has been cleared for transfer from Guantanamo since 2007.

"We've shed blood and treasure in a common cause in Iraq and in Afghanistan," Mitchell said, "And for our prime minister and for us to be treated in this way by America is insulting to their oldest ally, and I think it's extremely important that this matter is put to rest as soon as possible."

The former Conservative Party chief whip made the comments to VICE News on Thursday at the end of a cross-party delegation to the US to lobby for the transfer of the Saudi-born 46-year-old, who has been held without charge for 13 years.

They follow repeat assurances from the US that Aamer's case will be resolved. In January, President Obama told Prime Minister Cameron that Aamer's case would be prioritized. In April, US government officials said that he would be transferred within weeks. Mitchell was dismissive of these assurances, however.

"They said that six years ago and he's still inside," Mitchell told VICE News. "And he's not being released. It needs to happen."

Aamer was cleared for release to Saudi Arabia by the Bush administration in 2007, and by an interagency task force that reviewed his case in 2009. He has a British wife and four children, and UK officials fear that he will not be safe if he is deported to Saudi Arabia.

"They will hear me screaming in London if they try to drag me away from here to Saudi Arabia," Aamer wrote in a letter to his lawyer at human rights NGO Reprieve in 2013. "If they come in the middle of the night I will resist them every step of the way. I am going to London only; I wish to be with my wife and my kids. I have many concerns about Saudi and I am not going back to Saudi for many reasons. Most important is my family. They deserve me and I need to go back to them."

A sustained effort to transfer him to the UK — including a recent petition signed by more than 40,000 people — has so far failed to yield results, however.

"Whatever you think of Guantanamo — even if you approve of Guantanamo, which I don't — nevertheless, to be incarcerated for 13 years without charge, and to have been cleared twice… is appalling," said Mitchell.

Mitchell was one of two Conservative MPs in a delegation assembled by Reprieve to lobby for Aamer's release that included fellow Conservative David Davis and Labour MPs Jeremy Corbyn and Andy Slaughter.

Arriving in the US on Monday and returning Thursday, they met with senior senators including Dianne Feinstein and John McCain, who said they would look into the matter and "push it up the line," Mitchell said.

"We were there to lobby for his release and we were there to point out that the prime minister had been promised and nothing had happened. We were there speaking as a broad cross-section of parliament," he added.

However, Aamer's release can only be triggered by a note to Congress signed by the defense secretary — which provides Congress with 30 days' notice of his release.

"That note has not been issued," said Mitchell. He believes the delegation has made progress but says he can't quantify that or give an estimate of when Aamer will be released.

Mitchell agreed that the situation was embarrassing for Britain, "because we've asked that he be released or transferred to the UK, where we have the right security and care systems to address any aspects of this that needs to be addressed, and it hasn't happened."

A spokesman for the Office of the Special Envoy for Guantanamo Closure at the US State Department told VICE News that Aamer's case continued to be a priority.

"We are working to transfer each of the 57 detainees currently approved for transfer in a manner that protects our national security and is consistent with our humane treatment policies," he said. "We recognize the importance the UK government has placed on resolving Mr. Aamer's case in a timely manner, and we have made his case a priority."

A UK Foreign Office spokesperson said Aamer's case was a "high priority".

"We continue to raise Mr Aamer's case at the highest levels with the US authorities and welcome President Obama's continuing commitment to closing the detention facility at Guantanamo Bay," they said.

The last of nine British citizens and eight British residents to be released from Guantanamo, Aamer was born in Medina, Saudi Arabia. He studied in America and worked as a translator for the US Army during the first Gulf War.

Moving to London in 1996, he continued translating and married Zin Siddique, a British Muslim, fathering four children. They traveled to Afghanistan in 2001 where Aamer says he worked for an Islamic charity. After the US-led invasion, Aamer was captured by the Afghan Northern Alliance and handed over to the Americans, who offered bounties for alleged al Qaeda members.

The US has previously suggested that national security concerns remain over its release of Aamer. In 2007 Wikileaks released an assessment of Aamer prepared by a military analyst. It said that he was a close associate of Osama bin Laden and was a powerful figure at Guantanamo who could control other detainees to the point that he could persuade them to commit suicide. Aamer has always denied being associated with al Qaeda.