One of the country's leading human rights barristers is to resign her membership of the Liberal Democrats to express her outrage over the coalition government's backing for secret courts.

Dinah Rose QC successfully represented the British-resident Guantánamo detainee, Binyam Mohamed, in his battle to establish that British intelligence services were complicit in his "cruel and inhuman" treatment by the United States.

Citing her experience of secret hearings, she described Nick Clegg's support for the government's justice and security bill as a betrayal of the party's guiding principles.

Rose said her decision to resign had not been "taken lightly or without great sadness". She told the Observer: "The very first sentence of the Liberal Democrats' constitution states that they exist to build a 'fair, free, and open society'. The vote in favour of secret courts is an attack on the heart and soul of the party."

In her resignation statement, Rose said: "The right to a fair hearing, and the right to open justice, are among the most fundamental of all our basic constitutional rights. I just cannot see what purpose is served by the party, if it is prepared to support the bill. I have therefore decided, with great regret, to resign my party membership."

The Lib Dem leadership's decision to back plans to extend the use of secret hearings in civil courts, where matters of national security are at stake, has divided the party. In last week's Commons vote on the security and justice bill, in which Lib Dem leader and deputy prime minister Clegg ordered his MPs to back the controversial measures, party president Tim Farron and deputy leader Simon Hughes were among those who voted in favour of Labour amendments, which included the introduction of a public interest test for secret courts.

Ministers eventually defeated two attempts by Labour and coalition rebels to introduce extra safeguards into plans to allow civil courts to sit in secret, a move critics warn threatens the rule of law and could allow the security services to cover up involvement in abuse and torture.

Fury over the controversial measures will take centre stage at the Lib Dems' spring conference in Brighton, where Clegg will face a full-throttled attack after opponents within his party won the right to a debate on the issue.

The deputy prime minister attempted to reason with his critics, who claim Clegg has "betrayed" the founding principles of the party. He told a stormy question-and-answer session that he was unable to block the legislation for secret courts and that he believed his parliamentary party had dealt with the worst excesses of the bill going through parliament.

Clegg said: "Please don't allege that this government, with Liberal Democrats in it, is doing something abhorrently illiberal, which we are not."

Jo Shaw, a member of the party and a lawyer who has been leading opposition to the secret courts legislation, said Clegg had failed to reassure his party colleagues and she expected the conference to vote against the leadership on Sundaytoday.

She said: "These measures go against the fundamentals beliefs of Liberal Democrats." Julian Huppert MP, one of the Lib Dems who voted against the bill in the Commons last week, said his party's leadership had worked to improve the legislation but that it was still "not enough".

The failure of most Lib Dem MPs to ignore the party conference's overwhelming opposition to the plans has infuriated Lib Dem members. The Observer understands that several more high-profile supporters are considering resigning from the party.

Shami Chakrabarti, director of pressure group Liberty said: "When one of the pre-eminent human rights' QCs in the country resigns from the ranks, a leader would be wise to reflect on his direction."

She added: "Most Liberal Democrats are rightly horrified by the secret courts bill and astonished that the party that once made civil liberties its USP is now exchanging Mill for Kafka."

Rose has extensive experience of the Special Immigration Appeals Commission (SIAC), which deals with deportation cases that affect issues of national security and constitutes the most secret court in the English legal system. Pivotal cases Rose has been involved in include that of Bisher al-Rawi and other Guantánamo detainees in which they alleged ill-treatment during their detention.

The case led to a supreme court judgment banning secret hearings, known as closed material procedures (CMPs). That ban led the government to introduce the justice and security bill, which will expand the use of CMPs into civil courts.

Rose's resignation note added: "I believe that I am the only person who has acted in SIAC for the home secretary, as a special advocate, and for appellants.

"My conclusion, in common with the overwhelming majority of those who have acted as special advocates, is that CMPs cannot be fairly operated."