More than 50 members of the European parliament have signed an open letter to the US government raising concerns about the treatment of Bradley Manning, the US soldier in military detention for allegedly leaking classified US documents to the whistleblowing site WikiLeaks.

The call on the US government comes before a pre-trial hearing – Manning's first appearance in court – which begins on 16 December.

The MEPs said internal investigations into Manning's treatment in custody, which included solitary confinement for up to 23 hours a day, inspections by officers every five minutes from 5am onwards and removal of his clothes, had been marred by "clear conflicts of interest".

They call for US authorities to grant Juan Méndez, the UN special rapporteur on torture, access to Manning.

Mendez has made repeated requests for access to the military base where Manning is held, all of which have been refused by US authorities.

Manning was arrested in May 2010 after allegedly confessing to passing thousands of documents to WikiLeaks to computer hacker Adrian Lamo, who informed the authorities. He was charged with 12 offences under the uniform code in July 2010. An additional 22 charges were added in March 2011, following the publication of the Afghan and Iraq war logs, Guantánamo files and US embassy cables.

One charge, "aiding the enemy", carries the death penalty in the US, but army prosecutors have indicated they will not seek such a punishment in Manning's case.

The open letter from European parliamentarians, which follows another signed by several hundred US legal scholars, questioned the charges against Manning and warned that his pre-trial treatment may harm the UN's work elsewhere, "particularly its mandate to investigate allegations of torture and human rights abuses".

"In order to uphold the rights guaranteed to Bradley Manning under international human rights law and the US constitution, it is imperative that the United Nations special rapporteur be allowed to properly investigate evidence of rights abuses. PFC Manning has a right to be free from cruel and unusual punishment. People accused of crimes must not be subjected to any form of punishment before being brought to trial," they wrote.

"We certainly do not understand why an alleged whistleblower is being threatened with the death penalty, or the possibility of life in prison. We also question whether Bradley Manning's right to due process has been upheld, as he has now spent over 17 months in pre-trial confinement."

Five MEPs from the UK signed the open letter in support of Manning, who holds dual US and UK citizenships. They were Labour MEPs Richard Howitt and Derek Vaughan, Green MEPs Jean Lambert and Keith Taylor, and Plaid Cymru MEP Jill Evans.

Manning will attend an Article 32 hearing, the US military equivalent of a pre-trial hearing, on 16 December. This is expected to last five days. Manning's lawyer, David Coombs, has indicated he wishes to call 50 witnesses at the hearing, but military authorities are considered unlikely to grant such a request.

The Article 32 hearing then makes a recommendation to a general as to whether to proceed to a full trial.