The number of government websites is increasing despite a high-profile cull, Francis Maude has revealed.
The Cabinet Office minister said his officials were engaged in a "nightmarish game of 'splat the rat'".
"As soon as you knock one website on the head another one pops up," he told a government IT conference in London.
Mr Maude said all sites - including those for government agencies - would either be axed or moved into the gov.uk domain by the end of the year.
"There is no reason why every single bit of government should have its own unique web presence," he told the SPRINT 14 conference.
"It's complicated and it's expensive and we don't need to do it.
"Nearly 300 government websites will migrate to gov.uk in the coming year. Over a third have already moved but we have to bring in the remainder, bringing together government services at lower cost and consistent standards and simplicity for the citizen."
The government has been publishing a quarterly update of its progress in rationalising its websites since 2011.
The latest figures, to be revealed later this week, will show 19 websites have closed and a further 18 sites have been absorbed into gov.uk - but the total number of sites officials "are aware of" has gone up by 15 to 455, Mr Maude told the conference.
The previous Labour administration launched the first cull of government websites in 2007.
Most of the sites on the list of 551 sites earmarked for destruction have ceased to exist, some have become part of gov.uk and the domain name for others is now occupied by commercial websites. A few, however, still survive to this day, including the Highways Agency site for abnormal loads, the Equality and Human Rights Commission, and the NHS brand guidelines site.
Mr Maude claimed the gov.uk site, which last year won a Design Museum award, led the world in the simplicity and clarity of its design.
Minister and officials gave the assembled IT experts and civil servants at SPRINT 14, held to mark the half way point in the 400 working days the government gave itself to move 25 key services online, a live demonstration of how the latest services to be delivered online will work.
They include individual voter registration for elections, which will go live in England and Wales in summer 2014, and in Scotland following the referendum on independence in September 2014.
Plans to allow 41 million PAYE employees to view and manage their income tax records and information using a "digital tax account", were also showcased, along with a prison visitor booking system, a new online visa application system and a previously announced system for managing driving records.
Mr Maude also used the conference to set out further details of central government's move to "open source" software - ending its reliance on what he called the "oligopoly" of large IT contractors, such as Microsoft.
Document formats are set to be standardised across Whitehall, with software that can produce open-source files in the "open document format" (ODF), such as OpenOffice and Google Docs.
"Technical standards for document formats may not sound like the first shot in a revolution," he told the conference.
"But be in no doubt: the adoption of compulsory standards in government threatens to break open Whitehall's lock-in to proprietary formats. In turn we will open the door for a host of other software providers."
Mr Maude also hailed changes designed to increase the number of small and medium-sized companies winning public sector contracts, which he said had already gone up from 6% to 10% and would reach 25% by the next election.