A vigil will be held on the banks of the Thames to mark the 30th anniversary of the Marchioness pleasure boat disaster in which 51 people died.

Relatives and friends of the victims – whose average age was 22 – have been invited to carry candles and throw flower petals into the water on Monday evening. Police launches, along with those of the fire and rescue services, the Royal National Lifeboat Institution and the Port of London Authority (PLA) will gather at the spot on the river where the Marchioness sank after colliding with a dredger, the Bowbelle.

Christopher Chessun, the bishop of Southwark, will lead a service on the riverbank, and on Tuesday a memorial service will be held at Southwark Cathedral.

Meanwhile, the PLA said action on one outstanding safety issue, relating to older passenger boats still operating on the Thames, was “long overdue”.

The Maritime and Coastguard Agency (MCA) has proposed changes to ensure older ships match the standards of newer vessels relating to buoyancy in the event of a collision. A handful of “little ships” involved in the second world war Dunkirk evacuation could be taken off the water as a result.

Acknowledging the proposals were controversial, the MCA said: “We recognise that certain ships have intrinsic historical value and form an important part of the UK maritime heritage. That said, we cannot make a wholesale exemption for historic ships as a group when safety is concerned.”

The Marchioness was hired for a party to celebrate the 26th birthday of Antonio de Vasconcellos, who worked in a merchant bank. It left Embankment Pier at 1.25am on 20 August 1989 with 130 people on board.

At 1.46am, just after the pleasure boat had passed under Southwark Bridge, it was hit twice by the Bowbelle, which had been travelling behind it in the same direction. The Marchioness sank in less than half a minute.

According to Keith Fawkes-Underwood, who witnessed the collision from the south bank of the Thames, “the barge collided with the pleasure boat, hitting it in about its centre, then mounted it, pushing it under the water like a toy boat. Within a matter of about 20 seconds, the pleasure boat had totally disappeared underneath the water.”

Survivor of the Marchioness pleasure boat talks to Tim Ecott Read more

Most of those on board had no time to find life jackets, buoys or life rafts. Many were thrown into the water, but 24 bodies were found in the boat when it was raised. Some bodies were not recovered from the Thames for several days.

The day after the disaster, prime minister Margaret Thatcher returned early from a holiday in Austria, and transport minister Michael Portillo announced an investigation by the Marine Accident Investigation Branch. But the government swiftly ruled out a public inquiry.

The captain of the Bowbelle, Douglas Henderson – who had drunk six pints of beer during the afternoon before taking control of the dredger shortly after 1am – was charged with failing to have an effective lookout on the boat. However, two trials ended in a hung jury.

The victims’ families continued to press for a public inquiry into the disaster, which was eventually ordered by the Labour government in 2000. It concluded: “The basic cause of the collision is clear. It was the poor lookout on both vessels. Neither vessel saw the other in time to take action to avoid a collision.”

Victims’ families were also horrified to learn that, in more than 20 cases where bodies had putrified and bloated in the water, the Westminster coroner, Paul Knapman, instructed the hands of the deceased should be removed to be fingerprinted. Families were not told of the order.

Margaret Lockwood Croft, whose son Shaun, 26, died in the disaster, said the 30th anniversary would be significant, “but for us it’s like yesterday. Our children are always in our hearts.”

She helped to found and run the Marchioness Action Group, which campaigned for safety improvements on the Thames.

Among measures taken following the disaster were: the introduction of RNLI lifeboats at key locations; flashing lights on bridges to warn when larger vessels navigate through arches; a modern tracking system to give vessels the location of other boats and to allow the river authorities to monitor all river traffic; and grab chains and other lifesaving equipment along the riverbank in central London.

Ahead of this week’s anniversary, the Port of London Authority said in a statement: “Safety procedures on the Thames have been transformed since 1989.”

But, it added, “there is one remaining area – a small number of older passenger boats which are affected by the Maritime and Coastguard Agency proposals regarding buoyancy and post-damage survivability.

“The PLA strongly supports these proposals to ensure all passenger boats operating on the Thames in London meet modern standards for staying afloat after a collision. This is long overdue.”

Canon Michael Rawson, Southwark cathedral’s sub-dean, said: “For the families of those who died on the Marchioness and for survivors, the memories of the disaster 30 years ago remain as painful as they were on the day.

“Each year on the anniversary, the names of those who died are read out at services at Southwark Cathedral as we pray, too, for the healing of wounded memories. The cathedral continues to hold the thoughts and prayers of everyone affected, remembering the words engraved on the Marchioness memorial in the cathedral: ‘Many waters cannot quench love.’”