The government is actively looking into prime minister Boris Johnson’s idea of a road bridge linking Scotland and Northern Ireland, Downing Street has said, rejecting scepticism from engineers and criticism of the prime minister’s mixed record on delivering bridges.

“Government officials are carrying out work in relation to the idea of a bridge linking the GB mainland to Northern Ireland,” the prime minister’s spokesman told a media briefing. “There is a proper piece of work being carried out into the idea. It is reporting into No 10, but obviously we will take advice from wherever it is needed.” He gave no details on when the work would be completed.

Johnson, who as London mayor spearheaded the doomed garden bridge project which cost more than £50m without work even beginning, first raised the idea of the 20-mile-plus bridge to Larne in Northern Ireland in 2018.

Two routes have previously been floated; from Portpatrick to Larne or near Campbeltown to the Antrim coast. The Portpatrick route would be more than 20 miles across the Irish Sea.

He called again for the link during the Conservative leadership campaign, despite engineers expressing scepticism at the idea of building such a link over waters more than 300 metres deep in places, parts of which were used by the Ministry of Defence to dump more than 1m tonnes of obsolete munitions.

Asked about the bridge by the DUP MP Ian Paisley in the Commons in December, Johnson called it “a very interesting idea”, adding: “I advise him to watch this space and, indeed, to watch the space between the islands, because what he has said has not fallen on deaf ears.”

More than a decade ago the think tank the Centre for Cross Border Studies suggested a 21-mile bridge from Dumfries and Galloway could provide international rail links and ease the strain on air services.

At that time it estimated the cost of the scheme would be about £3.5bn. However, by last year the suggested price tag had risen considerably. Some experts suggested late last year that £15bn might be required for the project but, just months later, others have now said that £20bn would be a "conservative estimate.”

Johnson has long been an enthusiast for major infrastructure projects, but his visions have so far only enjoyed mixed results.

As London mayor, he pushed repeatedly for the garden bridge across the Thames, despite repeated warnings about its economic unfeasibility and the complication of securing planning consents, wasting £53.5m of funds, £43m of it taxpayers’ money.

An even more ambitious idea for a new London airport in the Thames estuary came to nothing. As mayor, Johnson did deliver a £60m cable car across the Thames in east London, but a supposed key transport link later turned out to have just four regular users.

Johnson’s spokesman rejected criticism of this record: “The prime minister is ambitious in terms of infrastructure projects. He’s looking at a wide range of schemes across the UK which could boost connectivity.”