A 33ft-long plan of the Titanic used during the inquiry into its sinking is going on public display for the first time in almost a century.

The detailed drawing, valued at £250,000, is marked in chalk with notes and arrows pointing out where survivors who gave evidence to the investigation believed the iceberg struck the ill-fated liner.

The unique plan has been privately owned since Lord Mersey’s British Board of Trade inquiry reported in the months after the April 1912 disaster.

But it is now up for sale again and, ahead of it going under the hammer at the end on next month, it will be on show in Belfast City Hall as part of an exhibition to mark this year’s 100th anniversary of the ship’s launch from the city’s docklands.

Alan Aldridge, from auctioneer Henry Aldridge & Son, said the plan was key to the UK investigation into the maritime tragedy – a probe which concluded the sinking was caused by striking an iceberg at excessive speed.

“As people were giving their evidence to the inquiry, they were asked to gauge where they thought the iceberg struck,” he said.

“So there are coloured marks on the plan and comments.

“The plan was very crucial to the inquiry, as important as anybody who gave evidence to it; and none of them are around anymore.” Mr Aldridge said almost 100 people testified during the 36-day inquiry.

“All people relevant to sinking took part, including many survivors,” he said. “There were officers, engineers, planners, representatives from White Star Line and Harland and Wolff shipyard in Belfast and passengers. Everybody who had an opinion contributed, including Ernest Shackleton the explorer as he was a mariner.”

The drawing – which is 32ft 6ins long and 4ft 8ins tall, and to a scale of 3/8 inch to the foot – is hand-drawn in Indian ink on a single piece of paper and shows all of Titanic’s main features, including passenger accommodation, crew quarters, cargo holds and boiler rooms.

The plan will go on public display over the Easter weekend.