Delays to construction work on London’s proposed garden bridge risk causing a major collision on the Thames on the scale of the 1989 Marchioness disaster, a leading marine civil engineer has warned.

Construction of the controversial 367-metre tree and plant-lined bridge designed by Thomas Heatherwick had been due to start this summer, but a series of hiccups with the project have put the schedule back to autumn at the earliest.

The government’s Marine Management Organisation (MMO) has now granted the Garden Bridge Trust, which is behind the project, a construction licence.

As part of a consultation process, however, the head of a leading marine engineering company wrote to say that delays mean the bridge will now coincide with other big projects involving large amounts of barge construction traffic on the Thames, the Architects Journal reported.

The publication obtained the letter, from Tim Beckett, the co-founder of leading marine civil engineering consultancy Beckett Rankine, under freedom of information laws.

It warns of a “hazardous juxtaposition of construction activities” between the bridge and projects including the construction of London’s so-called super sewer, the £4.2bn Thames Tideway Tunnel, or TTT, which will take away huge amounts of waste down the river.

Beckett’s letter noted that a previous risk assessment in 2014 had found that the area between Temple and South Bank where the bridge will sit, known as King’s Reach, had seen “significantly more vessel collisions and contacts than any other part of the river”.

He wrote: “The TTT will involve a tripling of large freight movements on King’s Reach while the Northern line extension and Fulham football ground works are due to add even more heavy freight movements. Meanwhile passenger boat movements are at a record level and still rising.

“During this unprecedented level of river traffic Blackfriars [Bridge] No 2 arch will be closed for the TTT works, further increasing navigational risk in King’s Reach.

‘To then add the two Garden Bridge cofferdams on top of all these other increased risks would, I believe, be an unnecessary and reckless risk. The likelihood of a major collision between a passenger vessel and a large freight vessel, such as happened 27 years ago with the Marchioness and Bowbelle, is simply too great.”

Fifty-one people drowned in 1989, when the Bowbelle, a dredger, struck the Marchioness, a pleasure cruiser hosting a party, at night.

Beckett recommended in his letter that the garden bridge works be pushed back to reduce such risks.

Separately, the head of the Thames Clippers commuter boat service said he did not feel there was a safety risk, but the volume of river traffic could affect his company’s services.

Sean Collins told the Architects Journal: “Given the clash of construction events within the area this is likely to seriously impact on the punctuality and reliability of our service and hope that this can be resolved through constructive consultation.”

In granting the construction licence, the MMO said the risks were acceptable if mitigation measures were in place, and that the Port of London Authority had no objections to the bridge on safety grounds.

A Garden Bridge Trust spokeswoman said: “We have been in extensive dialogue with the TTT to ensure that our respective works are fully integrated with no construction clashes.”