Secret trials and withholding evidence are reminiscent of "repressive regimes and undemocratic societies", the legal profession warns in a letter opposing the government's justice and security bill.

Ken Clarke's plans will erode core principles of justice and "fatally undermine the courtroom as an independent and objective forum", according to the organisations representing solicitors and barristers. The UK's international reputation for "fair justice" will be significantly damaged, they say.

A senior Liberal Democrat source has signalled that the party, whose annual conference rejected the bill, expects changes to the controversial proposals during their passage through parliament to "improve the legislation". The intervention by the Law Society and the Bar Council comes before next week's pivotal vote on the regulations, which extend secret hearings, known as closed material procedures (CMPs), into the main civil courts. It follows amendments by the Cabinet Office enabling even the supreme court to go into secret session, which was never previously contemplated. That change would allow the UK's most senior judges, who normally examine competing legal precedents, to hear intercept evidence or classified intelligence barred to claimants.

A last-minute concession by the government on Friday withdrew powers in the bill that would have allowed secret hearings to be extended into other courts solely by ministerial decree.

Human rights groups and prominent lawyers have condemned part two of the bill, which expands the role of secret courts. Opponents fear the powers will enable ministers, rather than judges, to manipulate the way evidence is withheld or presented, depriving litigants of a fair trial. The government maintains the changes will enable judges to try a greater range of national security cases and prevent claims being settled out of court.

The letter from Lucy Scott-Moncrieff, president of the Law Society of England and Wales, and Michael Todd QC, chairman of the Bar Council, has been sent to Clarke, the minister without portfolio who is steering the justice and security bill through the Commons, and Lord Wallace of Tankerness, the Liberal Democrat peer who is guiding it through the Lords. It states: "[Close Material Procedures] depart from an essential principle of natural justice … that all parties are entitled to see and challenge all of the evidence relied upon before the court, and to combat that evidence by calling evidence of their own.

"In addition, [they] also undermine the principle that public justice should be dispensed in public and will weaken fair trial guarantees and the principle of equality of arms. These are both essential concepts of the rule of law.

"Secret trials and non-disclosure of evidence are potential characteristics of repressive regimes and undemocratic societies.

"Whilst HM government rightly takes a strong stance in respect of the importance of the rule of law globally, we fear that if passed, this bill will adversely affect the UK's international reputation for fair justice."

The letter continues: "It will be impossible for lawyers to advise their clients in their best interests if they are not privy to the information being used against them in court and are able to discuss this with those clients. Again, we believe this breaches a fundamental right of defendants in a just society.

"We believe that the plans for secret courts erode core principles of our civil justice system and will fatally undermine the courtroom as an independent and objective forum in which allegations of wrongdoing can be fairly tested and where the government and others can be transparently held to account."

A government minister has admitted that a key purpose of expanding secret courts is to introduce more intercept evidence into trials.

In a debate this summer, Wallace told the Lords that if information obtained through sources such as telephone tapping could not be deployed, "it would narrow the scope of material available in the case and could undermine the wider purpose of CMPs, which is to make available as much material, helpful or unhelpful to the government, as possible".

Civil liberties groups, which support the use of intercept evidence openly in court, object to its exploitation in the context of secret courts where it cannot be fully challenged. Isabella Sankey, Liberty's director of policy, said: "Government plans to lock claimants and the public out of ordinary courts will allow wiretap content to be quietly passed to judges out of sight and immune from challenge.

"Liberty has long pushed for intercept material to be safely put before the courts, but these proposals will stack the odds firmly in the government's favour, co-opting judges into a spook-authored, clandestine new world. What clearer sign of an ambition to put ministers and securocrats above the law?"

Opponents are divided over whether to vote down part two of the bill, expanding secret courts, or unite behind an amendment that substantially narrows the scope of its effect.

Sadiq Khan, Labour's justice spokesman, has indicated his party would prefer to adopt recommendations from parliament's joint committee on human rights (JCHR) restricting "national security'' only to protecting the identity of UK intelligence officers, their sources and security material provided by foreign allies.

Liberal Democrats, whose annual conference overwhelmingly rejected the bill in defiance of the party leadership, face an acute political dilemma. A senior Lib Dem source told the Guardian: "Our conference clearly demonstrated the strength of feeling in the party about the issue. The Liberal Democrats have always placed the utmost importance on the defence of civil liberties.

"The JCHR report on the bill was an important contribution to the debate and the government has said that it will reflect on it. We are very much in listening mode but we are also in a coalition and things have to be agreed on both sides.

"The bill has a long way to go in its passage through parliament during which the Liberal Democrats will ensure that a proper debate is had and there will have to be changes to improve the legislation."

The National Union of Journalists says it is "alarmed" by the proposals. Michelle Stanistreet, the organisation's general secretary said: "[It] will close down reportage on civil proceedings and court cases. This will undermine a fundamental constitutional right: the right to open justice."

Announcing the latest amendment to the justice and security bill, Clarke said: "We have removed the secretary of state's power to extend the scope of the bill entirely. The bill is now focused even more tightly on the small but increasing number of national security sensitive civil cases which cannot currently be heard in our courts.

"It will set to rest fears raised by the [JCHR] that the order-making power could have been misused, and put beyond any doubt whatsoever the fact that inquests are beyond the scope of the bill.

"This bill ends the current bar on judges taking national security evidence into account when reaching a judgment. By ensuring that evidence relating to the most secret actions of the state is argued over before a judge in a closed hearing we will ensure more scrutiny of our intelligence and security agencies, not less."