The Attorney-General is accusing them of scandalising the Singapore judiciary, and on Tuesday the High Court gave the go-ahead to start proceedings against the trio for contempt of court.

The accused are: Singapore Democratic Party (SDP) assistant secretary-general John Tan Liang Joo, 47, full time national serviceman Muhammad Shafi'ie Syahmi Sariman and activist Isrizal Mohamed Isa.

They wore the T-shirts during a hearing from May 26 to 28 that involved the SDP, its chief Chee Soon Juan and his sister Siok Chin, a member of SDP's central executive committee.

The hearing before Justice Belinda Ang in Court 4B was to assess defamation damages the party and the Chees had to pay Prime Minister Lee Hsien Loong and Minister Mentor Lee Kuan Yew.

Tan allegedly said 'This is a kangaroo court' to MM Lee when the minister walked past him outside Court 4B, said a statement on the website of the A-G Chambers on Tuesday.

In a separate statement to the media, the A-G said the three men had 'scandalised the Singapore Judiciary by publicly wearing identical white T-shirts, imprinted with a palm-sized picture of a kangaroo dressed in a judge's gown, within and in the vicinity of the New Supreme Court Building.'

By this, they meant to imply that the Court was a kangaroo court, the AG added.

A kangaroo court is generally understood as being a court characterised by unauthorised or irregular procedures, or sham and unfair legal proceedings, said the website statement.

It also said that Tan, as the SDP's assistant secretary-general, was also responsible for the appearance of an article, 'Police question activists over kangaroo T-shirts', as well as a photograph of the three men in the T-shirts on the SDP website on July 27.

'The article and the photograph...were meant to give wider publicity to their allegation that the Court was a kangaroo court,' said the AG website statement.

The AG accused them of engaging in a 'deliberate and calculated course of action to impugn the reputation of and undermine public confidence in the Singapore Judiciary, and to lower its authority in the administration of justice in Singapore.'

The High Court's approval to start contempt proceedings is the first of a two-stage process.

In the next step, the three men will be officially notified of the lawsuit, and a hearing date will be set for both sides to present their arguments in open court.

Tan and Muhammad Shafi'ie are also facing separate charges in court, with 17 others, for their involvement in an illegal assembly and march on March 15.