Microsoft will spend more than $440 million (£295.2m) in licensing fees and sales and marketing costs over the next five years to keep up its end of the historic Linux agreement it made with Novell to support open source on Windows, according to US Securities and Exchange Commission (SEC) documents filed on Tuesday.

Under the deal, Microsoft has agreed to offer marketing and sales support for Novell's SUSE Linux and to co-develop technologies that will help customers integrate the competing operating systems.

But the most controversial component of the deal may prove to be the patent cooperation agreement under which Microsoft has promised not to launch patent lawsuits against Novell customers.

Linux advocates maintain that it is effectively a patent cross-licensing agreement between Novell and Microsoft. Such an agreement would be in violation of Linux's software license, the GNU General Public License, which does not allow distributors to enter into exclusive agreements with patent holders.

But in a note published yesterday, Novell said its agreement with Microsoft only amounts to a "covenant" between the two companies’ customers.

"Novell's customers receive a covenant not to sue directly from Microsoft," the company said on its website. "We have not agreed with Microsoft to any condition that would contradict the conditions of the GPL, and we are in full compliance."

But free software advocate Bruce Perens said that this argument was unlikely to hold up in court. He pointed to the fact that Novell is paying Microsoft more than $40m as its part of the patent agreement only makes it obvious that the deal amounts to patent cross-licensing. "The financial consideration makes it even more clear that there is a patent licensing agreement for money going on here," he said.

The most likely candidate to sue Novell or Microsoft over an alleged GPL violation is the Free Software Foundation (FSF), which claims copyright to significant portions of the Linux operating system, Perens said.

FSF General Counsel Eben Moglen said he was looking into the Microsoft-Novell deal but could not comment on the documents released by Novell.

In Tuesday's SEC filings, Novell said Microsoft will make an initial payment of $240m for the SUSE certificates and will also spend a total of $34m on sales efforts, as well as $12m per year on marketing over the life of the five-year agreement.

Under the patent agreement, Microsoft will pay Novell $108m up front and Novell will pay Microsoft a percentage of revenue that adds up to no less than $40m over five years.

Microsoft has also agreed that it will not engage in a similar certificate give-away with another Linux distributor for the next three years.

The two companies announced the surprise agreement late last week, billing it as an effort to "bridge the divide between open source and proprietary source software," according to Microsoft chief executive Steve Ballmer.

Microsoft has also agreed to distribute 70,000 SUSE Linux subscription certificates so Microsoft customers can get SUSE updates and technical support from Novell.