The government is in danger of creating a "them and us" situation by digitising public services, a report warns.
Ministers say 82% of "transactions" can be carried out online, as that is roughly the proportion of the population which uses the internet.
But the National Audit Office argued that the percentage of people able to access some services, such as those used by elderly people, was lower.
It called for "continued access" to face-to-face and telephone services.
The government said it was continuing to offer help to users and promised to create websites "so good (that) people will prefer to use them".
The coalition has moved most government services to the single gov.uk address, after Whitehall departments set up their own sites in a more piecemeal fashion. Other bodies are expected to follow by March next year.
It estimates that making services "digital by default" may save up to £1.2bn during the current parliament, with future savings potentially reaching £1.8bn a year in the longer term.
A study has put the average cost of face-to-face transactions at £8.62 each, those via telephone at £2.83 and those via a website at 15 pence.more