SNP ministers are "unlikely" to meet their target of providing every Scottish home with access to superfast broadband by their 2021 target, an official audit has warned.
Audit Scotland concluded it would be "difficult" to deliver the R100 programme for every home and business to have access to speeds of 30mbps by the end of that year.
The report said ministers expect to extend access to "most" of the 147,000 premises it is targeting "but it is unlikely to reach them all."
Contracts to internet providers are not being awarded until early next year, it said, leaving less than three years for "the most complex premises" to be reached.
Although the Scottish Government has set aside £600 million, the report warned additional funds may be needed and highlighted a lack of clarity over how the roll-out will be achieved.
It said ministers are considering other options involving mobile, wireless or satellite technologies, which are cheaper to install than fibre lines but less reliable and more expensive for consumers.
The auditors said this is "likely" to involve households in rural areas applying for vouchers they can spend to help fund access to superfast broadband.
They praised the SNP government for hitting its previous target to provide fibre broadband acess to 95 per cent of premises by the end of 2017, saying without public investment this proportion would be only two-thirds.
It is not yet clear how the Scottish Government is going to fulfil its pledge to deliver superfast broadband to everyone by the end of 2021
Fraser McKinlay, Audit Scotland director of performance audit and best value
Overall connection speeds have increased across Scotland but around a quarter of rural areas still cannot receive a basic speed of 10mbps.
Paul Wheelhouse, the SNP's Connectivity Minister, said the report demonstrated the "excellent progress made" and insisted he was confident the R100 deadline would be met.
But Holyrood's opposition parties accused the Nationalists of making "glacial" progress by taking years to award key contracts.
Although superfast broadband is a policy area reserved to Westminster, the devolved administrations have responsibility for areas where providers such as BT and Virgin are unwilling to provide it on a commercial basis.
Unveiling the report, Fraser McKinlay, Audit Scotland director of performance audit and best value, said: "Good progress has been made to date but the toughest hurdle remains - to extend the benefits to everyone, particularly remote and rural communities.
"As well as being the toughest hurdle, it is not yet clear how the Scottish Government is going to fulfil its pledge to deliver superfast broadband to everyone by the end of 2021."
Auditors found up to March 2018, the Scottish Government and Highlands and Island Enterprise (HIE) had paid £259 million to BT for broadband roll-out.
Lower costs and higher take-up is expected to enable around 60,000 more premises to be reached than originally planned.
But only 13 of the 63 initiatives financed by agency Commuinity Broadband Scotland (CBS) succeeded, with a lack of specialist skills, poor communication and complex tendering requirements causing lengthy delays and failed procurements.
Community groups told auditors this has undermined their confidence in the Scottish Government and Highlands and Islands Enterprise to support rural broadband.
Among the report's recommendations were for ministers to publish "clear timetables" so rural communities know when they can expect the R100 programme to reach them.
Jamie Greene, the Tories' infrastructure spokesman, said the SNP failing to meet the 2021 target "could cost businesses millions."
He added: "The fact is that Scotland has fallen behind England in terms of access and speed, indeed it took the SNP nearly four years to even open the procurement process, let alone get a contract signed."
Mike Rumbles, a Liberal Democrat MSP, said: "The fact of the matter is that the Scottish Government’s contribution to broadband development has so far been less than glorious and progress has been glacial."
Mr Wheelhouse admitted the R100 deadline "won't be easy to deliver" but said: "We expect to award the contracts in 2019 and remain confident that delivery of these, alongside other interventions, will allow the target to be met."