Solicitors for one of the Bloody Sunday families have contacted the Attorney General for Northern Ireland claiming Defence Secretary Gavin Williamson may have jeopardised the chances of a fair trial of a soldier due to be charged with murder.

Lawyers acting for the family of William Nash, who were told on Thursday that no soldier will face prosecution over his death, argued Mr Williamson’s comments may be in contempt of court.

One soldier, known only as F, is to be charged with two murders and four attempted murders on Bloody Sunday.

Following the decision announced by the Public Prosecution Service in Northern Ireland on Thursday, Mr Williamson confirmed the Ministry of Defence would support Soldier F and pay all legal costs.

He said: “We are indebted to those soldiers who served with courage and distinction to bring peace to Northern Ireland.

“The welfare of our former service personnel is of the utmost importance and we will offer full legal and pastoral support to the individual affected by today’s decision.

“This includes funding all his legal costs and providing welfare support.

“The Ministry of Defence is working across Government to drive through a new package of safeguards to ensure our Armed Forces are not unfairly treated.

“And the Government will urgently reform the system for dealing with legacy issues. Our serving and former personnel cannot live in constant fear of prosecution.”

Lawyers have taken issue with the phrase referring to efforts to ensure members of the armed forces “are not unfairly treated”.

In their letter to the attorney general they say it appears “Mr Williamson seeks to assert that the decision to prosecute, and the subsequent prosecution, is in some way unfair or that the accused has been, or is being, unfairly treated”.

Solicitor Darragh Mackin, of Phoenix Law, said: “Not only has the Secretary of State threatened the fairness of the judicial process, he has also shown a blatant disregard for our client and the other families affected by the events of Bloody Sunday.

“Mr Williamson seems to have forgotten that as an MP he has responsibilities to all citizens and not just the armed forces.

“Many lives in Derry were destroyed on Bloody Sunday and he would do well to be mindful of that.

“It is vital the legal process be allowed to function without attempts to influence it.”

MPs have campaigned for a statute of limitations which would prevent troops from being prosecuted for serving their country, including in Northern Ireland.

Speaking recently in a BBC interview, Mr Williamson was asked whether there should be a time limit on prosecutions of service personnel.

He said: “Absolutely, to ensure that we don’t have spurious prosecutions.”

Mr Williamson told Political Thinking With Nick Robinson’s BBC Radio 4 podcast: “No-one in the Armed Forces wants to be above the law, but what we did need to do is ensure that they do have the protection so that they don’t feel under threat.

“It’s not just about Northern Ireland, but about Iraq and Afghanistan, conflicts before that and in the future.”

In response to a question about whether that would make a difference to Bloody Sunday, he continued: “Sadly, I don’t think that will come in time.

“I think we have to ask a real question as to Northern Ireland has moved on. There’s been so much progress – we’ve got to look to the future, not at the past.”

John Kelly, whose 17-year-old brother Michael was killed, told reporters at a press conference at the Guildhall after families learned of news of Soldier F’s prosecution, that the attorney general should decide if the Defence Secretary or other politicians have broken the law in their comments.

Mr Kelly said: “If they have, they should be charged.

“They cannot attempt to interfere in a judicial process just because they don’t like it, or because their voters don’t like it.”

Grainne Teggart, from Amnesty International, also accused Mr Williamson of being disrespectful by not mentioning the Bloody Sunday families in Thursday’s statement.

She said: “The comments from Gavin Williamson are deeply concerning, and the absence in his statement of any reference to the Bloody Sunday families for the unjustified and unjustifiable killings of 1972 is disrespectful.

“We call on the UK Government to ensure there are no barriers to justice and make clear there will be no de facto amnesty for human rights abuses, including those committed by security forces.”

A Government spokesman said the safeguards referred to are “for how we deal with wider legacy issues, not specific legal cases, which it would be inappropriate to comment on”.