Transport Scotland has said plans to reopen a rail link between the Borders and Edinburgh are "on track" despite a member of one bid group withdrawing.
It follows concerns raised by the news that American firm Fluor was no longer seeking to be part of the project.
A spokeswoman said the company was part of one of three bidders for the £230m construction scheme.
She said it was not unusual for a consortium member to pull out and works were still expected to start in 2011.
Fluor was part of the New Borders Railway group, one of three bidders invited to submit tenders for the contract earlier this year.
The others were BAM and IMCD.
The Transport Scotland spokeswoman said it hoped to hear from New Borders Railway soon on a "potential replacement" for Fluor.
"The competition for Borders railway remains on track and we expect to begin construction in 2011 and have an operational railway up and running for communities in Midlothian and the Scottish Borders in 2014," she said.
"We are aware that the New Borders Railway consortium - one of the three consortia chosen from an original shortlist of five - is currently considering the replacement of one of its grouping and we expect to hear from it shortly.
"It should be noted that change to the commercial priorities of individual bidders during procurement is not uncommon, particularly on large scale projects where the timescales for tendering and construction are lengthy."
Tweeddale, Ettrick and Lauderdale Lib Dem MSP Jeremy Purvis said the situation was "concerning" and urged the Scottish government to move quickly to secure a contractor for the line.
"Under the current schedule there is still a very long wait of a full year before there is a final contractor in place and that does leave the door open to further concern," he said.
"The Scottish government wants to have the main contractor in place for the new Forth crossing before the Holyrood elections in May and I want them to do the same for the Borders railway.
"The railway is critical to the future needs of the Borders economy and this is the best time to be building infrastructure."