The council had been involved in its own Linux trials last year with the Net Project group but council officers decided such a major migration would pose "unacceptable levels of risk" to council services.
Nor will Newham take part in the Office of Government Commerce open source trials with IBM, instead focusing its energies on talks with Microsoft.
Richard Steel, head of ICT at Newham, told silicon.com that while the council will continue to monitor open source, the time is not right for such a switch given Newham's complex IT environment.
"We got to the point where we understood the market for open source enough to make a decision. At this point in time the risks were too great," he said. "In terms of moving forward in other areas such as groupware, there were significant difficulties in migrating."
Steel said it is easier for "greenfield" IT sites to move to Linux than those with established and heterogeneous set-ups.
Newham currently has around 5,000 desktops running various operating systems and different versions of Microsoft Office but will be looking to standardise on Windows XP if negotiations with Microsoft go well.
"The goal from the start has been standardisation and that is part of the negotiations with Microsoft," said Steel. "It is interesting to see how Microsoft has reacted in the last six months. They have a much better understanding of the environment we have and the strategy and context, and that open source has a role in that."