LONDON (DPA) - British Prime Minister Boris Johnson on Thursday (Sept 12) said he is considering a plan to build a 35km-long bridge across the Irish Sea from Scotland to Northern Ireland.

The idea of a bridge linking south-western Scotland with Northern Ireland has been discussed for many years, but reports on Wednesday suggested that Johnson has asked officials to begin feasibility studies.

"I was talking last night about building a bridge from (near) Stranraer in Scotland to Larne in Northern Ireland, Johnson told reporters on Thursday during a meeting with children in London.

"That would be very good," he said.

"It would only cost about £15 billion (S$25 billion)."

Broadcaster Channel 4 said it had seen documents asking officials to "explore the possibility" of building the bridge.

It quoted the documents as saying Johnson wanted an assessment of the risks of the project and of "where this money could come from."

The risks appeared to include "munitions in the Irish Sea" from World War II, the documents said.

The idea of a bridge across the sea was explored in the 19th century, and former Conservative prime minister Margaret Thatcher's government examined plans for a road crossing in the 1980s, the broadcaster said.

Northern Ireland's Democratic Unionist Party, which has close ties to Johnson's Conservatives, has supported the idea of a bridge since at least 2015.

But some experts have raised concerns over the depth of the sea and the frequent storms that hit it.