Railtrack has replaced Brown & Root's project director with its own executive as part of a massive management shake-up to get the project finished on time.
The moves follow rumours that the project is falling behind schedule, although Railtrack denied West Coast Main Line (WCML) was suffering delays.
The decision ends a planned "partnering agreement" between the two sides, announced last May, which was never ratified. The future role of Brown & Root, which is part of the pounds 5bn-a-year Halliburton Company, has not yet been defined, Railtrack said.
"We aren't saying that Brown & Root won't have a role, purely that we are reassessing how we take the project forward," said a spokeswoman.
She said Ian McPherson, B&R project director, had been replaced by Tony Fletcher, Railtrack's director of capital programmes. Mr McPherson has returned to B&R.
The company has split the scheme into two divisions, commercial and project delivery. "We are now at the delivery stage and we want to accelerate quickly and smoothly and get the thing built," she added.
Railtrack still has WCML partnership agreements with Alstom, for signalling and train control systems, and with Balfour Beatty and Westinghouse for the remodelling at Euston.
B&R has worked with Railtrack since the rail infrastructure company was formed in 1994, when B&R led a consortium preparing the feasibility study for the modernisation programme.
Railtrack is committed to achieving 125mph running between London and Glasgow by 2002 and 140mph by 2005. If it misses the deadlines, it faces huge financial penalties from Virgin Trains, which is investing pounds 2.25bn on 131 new trains.
Railtrack is also under pressure from the Rail Regulator to show that it can deliver major improvements to the network.
B&R was unavailable for comment but a spokesman told New Civil Engineer magazine: "There is nothing untoward, it is simply a reorganisation. It is part of the way big projects are run. They reorganise from time to time."