The plan to use Bupa's Redwood Hospital at Redhill, Surrey, as a diagnostic and treatment centre was announced in December last year.
Shadow Health Secretary Dr Liam Fox said, despite ministers' promises then, nothing had actually happened.
But a Bupa spokeswoman told BBC News Online that, as promised in December, the centre would be fully up and running by the end of this year.
She said it was already treating about 100 NHS patients a month for some orthopaedic and endoscopic procedures.
Dr Fox told BBC Radio 4's Today programme: "They said it was a done and dusted deal. They said that they were at the final stages - we would soon be having patients treated.
"Now we find that nothing has actually happened at all."
He said the reason for the delay was divisions between Chancellor Gordon Brown and Health Secretary Alan Milburn.
'No major problems'
Bupa admitted detailed contract negotiations were still taking place.
The spokeswoman added: "The contract, as it stands, has not been signed, but there are lots of things that have already been happening.
"We have started building work that's needed, we're preparing theatre lists. So it's pretty much up and running as we'd expected."
She said: "The negotiations have to be thorough and they have to be transparent.
"We couldn't have got this far if there were any major problems."
Fast-track treatment
A Department of Health spokesperson said: "The negotiations with Bupa are going well and we hope will be concluded satisfactorily within the coming weeks.
"However, these negotiations are both ground breaking and complex and it's important that we get the right deal for all parties involved.
"We did make it clear right from the start that we would not proceed until we were able to ensure getting good value for money for the taxpayer and the highest clinical standards for patients."
Gatwick Park, another Bupa hospital close to Redhill, has been taking all the companies insurance and self-pay patients since the beginning of the year, freeing up Redwood Hospital for NHS patients.
Under the arrangement, the hospital will perform 5,000 routine operations such as hip and knee replacements each year for the NHS.
It will be one of the proposed fast-track diagnostic and treatment centres. Earlier this month, the government announced 10 such centres.