An engineering company accusing HS2 of having conflicts of interest has called for the bidding competition for a major contract, which includes the South Yorkshire stretch of the route, to be re-run.
But Mace's demand was refused by HS2, which insisted there are 'no plans' to repeat the process to decide who will design Phase 2b of the high speed rail line, running from Crewe to Manchester and Birmingham to Leeds, including the South Yorkshire stretch of the route.
Mace believes competitor CH2M should not have been handed a £170 million deal last month because of its close links with HS2.
The contract signing was delayed after Mace threatened to sue due to the alleged conflicts of interest.
On Wednesday CH2M withdrew from carrying out the work but insisted it took ''all appropriate measures' in order to 'ensure the integrity of the procurement process'.
HS2 said it has opened discussions with the runner-up in the process, Bechtel.
A spokeswoman from Mace said: "After many weeks of waiting, we look forward to seeing HS2 Ltd tomorrow to finally hear their answers to our numerous questions about how the entire procurement process has been conducted.
"It's important to note that conflicts of interest was only ever one element of our concerns.
"With so many questions being asked by the public and Parliament, the only sensible thing to do now is to look at the whole tender process again."
CH2M has been involved with HS2 Ltd since 2012 and was awarded a £350 million deal to develop Phase 1 of the line from London to Birmingham. Mark Thurston, HS2 Ltd's new chief executive, is a former CH2M employee, as was his temporary predecessor Roy Hill.
A total of 25 HS2 staff have previously worked for CH2M, while 37 of the engineering company's employees are on secondment to HS2.
Mace has not yet launched legal action, but has instructed procurement specialist Michael Bowsher QC of Monckton Chambers.
The Mace spokeswoman said: " In our 26-year history, working on some of the UK's most iconic projects, we have never brought a case to the High Court or seen a procurement process run like this.
"We continue to closely review our next steps with our legal team and don't rule anything out."
Shadow transport secretary Andy McDonald claimed the situation "stinks to high heaven" during transport questions in the House of Commons.
Tory MP Cheryl Gillan said confidence in HS2 had been 'called seriously into question' as she asked Transport Secretary Chris Grayling if CH2M had jumped 'or were they pushed'.
She also called for a full inquiry to take place before any more contracts are awarded as part of the project.
Mr Grayling said: "CH2M have done the right thing in taking a step back, having identified a problem that would have called into question whether they could and should operate this contract.
"This was not some massive misdemeanour. It was an error in process which has caused them to take a step back."
The project suffered a further blow on Thursday as it was confirmed the director-general of the Department for Transport's (DfT) High Speed 2 Group had resigned. David Prout, who began the role in January 2013, has been appointed p ro vice-chancellor for planning and resources at Oxford University.
He will leave his role at the DfT in June. Phase 1 of the HS2 scheme is scheduled to open in December 2026, with a second Y-shaped phase launching in two stages.
Phase 2a from the West Midlands to Crewe will open in 2027 and Phase 2b will begin operation in 2033. Legislation for Phase 1 passed its final hurdle in Parliament in January and construction work is set to begin in the coming weeks.