A rail contractor has been suspended from bidding for new business until its workforce safety performance improves.
The suspension involves Carillion Rail, which deals with track renewals, and follows a number of workplace accidents.
Network Rail said Carillion's existing contracts would remain. Carillion said it was "disappointed" at NR's decision.
NR said it had been working with Carillion on a workforce safety improvement plan, but had been disappointed.
Simon Kirby, NR's director of major projects and investments, added: "We are raising the bar on workforce safety and Carillion Rail must improve its record.
"We want to give Carillion Rail the opportunity to focus on improving safety performance while delivering current contracts, rather than concentrating on bidding for new business.
"This is simply a precautionary measure designed to raise standards even further; this is not a response to any specific safety risk to the workforce or to passengers."
Saying it was committed to the highest safety standards, Carillion acknowledged that there was "an increase in less serious workplace accident in Carillion Rail between mid-June and mid-July, during which time we were under-taking a major restructuring of our rail business".
The company added that it considered NR's decision as "wholly disproportionate to our overall performance in respect of workplace safety".
Bob Crow, general secretary of the Rail Maritime and Transport union, said: "Network Rail bears ultimate responsibility for safety, and they can't expect us to swallow a bland assurance that there is no specific risk even though they have barred Carillion from new work.
"If Carillion's safety regime is not good enough, then surely its existing contracts must be brought back in-house.
"Track maintenance was taken back in-house by Network Rail for safety and efficiency reasons in 2003, and both have improved dramatically.
"Yet on the renewals side, the fragmentation of privatisation, with the same tangle of contractors, subcontractors, one-man-and-a-dog outfits, and the dismantling of the essential central command system, is still in place."