A secretive downgrading of the HS2 high-speed rail link would be "totally unacceptable", Labour has said.
Shadow transport secretary Lilian Greenwood said there have been suggestions that the Government may cut Sheffield out of its plans as well as a link to Stoke and Stafford.
She spoke after Transport Secretary Patrick McLoughlin said passengers would soon be able to access a high-speed rail link to Sheffield.
Ms Greenwood highlighted reports that phase two of HS2 is under review, with critics invited to the Treasury for discussions and business minister Anna Soubry saying Sheffield could "miss out".
The Labour MP went on: "Two months ago this House voted overwhelmingly in favour of HS2 on a specific understanding of the project.
"Of course costs must be kept under control, but it would be totally unacceptable if the plans for high-speed rail in the Midlands and the North were downgraded by some unaccountable and secretive review."
Earlier Mr McLoughlin included Sheffield in his speech when discussing the rail link.
The Transport Secretary said: "I am glad that I'll no longer only be able to get a high-speed train from London to Paris or Brussels, but will be able to soon get them to Manchester, Birmingham, Leeds or Sheffield."
Clive Betts, Labour MP for Sheffield South East, said ministers need to spell out exactly what the plans are for the city.
He said: "We still want firmer assurances from Government that the station for HS2 is going to be in Sheffield and that HS3 is going to link Sheffield as well as Leeds across the Pennines, as well as a commitment to the review of the Pennine Tunnel which many of us are really interested in."