Those using IE7, Safari 3, Firefox 3.5 and their predecessors to view Gmail, Google Calendar, Talk, Docs and Sites will then lose some functions.
Eventually, it warned, these web services will stop working for those sticking with older browsers.
The move is part of a trend to stop the use of ageing browsers which can be insecure and not sophisticated enough to handle the latest web technologies.
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Statistics on browser versions gathered by StatCounter suggest about 17% need to change in the light of Google's decision.
Google made its announcement in a blogpost saying its engineers were keen to make use of the latest capabilities in browsers, and that required support for HTML5 technology.
As a result, from 1 August, Google will only support what it calls "modern browsers". By this it means the latest versions and major prior releases of Chrome, Firefox, Internet Explorer and Safari.
As new versions of these are released, Google will get its web services working with that and then drop support for the third-oldest version.
Support in this sense means that Google will only do compatibility testing with more up-to-date browsers. It will not carry out tests with older programmes and can make no guarantees that web services will work with them.
Concluding the blogpost, Venkat Panchapakesan, vice president of engineering at Google, wrote: "These new browsers are more than just a modern convenience, they are a necessity for what the future holds."
In mid-May, Mozilla, which oversees development of Firefox, kicked off a plan to get the 12 million or so people using version 3.5 of its browser to update.
It said it was "frustrated" with efforts to get people to upgrade and had taken a series of steps to force change.
It used pop-up screens, adverts, re-directs and updates to steer people towards more recent versions of Firefox.
Figures gathered by Mozilla suggest the campaign has had some success as the number of users on Firefox 3.5 has now dropped to about one million.
Microsoft's campaign to stop people using Internet Explorer 6 is one of the longest running upgrade efforts.
The software giant has used its automatic update system to get newer versions of its browser out to many users.
However, many companies prefer not to use this system and that has meant IE6 clinging on in some firms and nations.
Globally about 11% of browsers are IE6, suggest figures compiled by Microsoft, and there is a wide variation around the world.
About 34% of Chinese net users are on IE6, as are 22.3% of South Koreans and 11.6% of Vietnamese people.