Thousands of NHS patients in the West Midlands are wait-ing longer than the Govern-ment's 13-week maximum to h ave their condition diagnosed.
Some have been waiting more than six months.
Figures revealing the waiting period between seeing a GP and diagnostic tests were published yesterday for the first time.
Ministers described them as a "hidden waiting list".
From April next year, health trusts will be expected to ensure patients receive diagnostic tests within 13 weeks of seeing a GP, and are treated within 18 weeks.
Health Minister Andy Burnham said the Department of Health did not yet know exactly how many patients were waiting longer than 18 weeks - but preliminary figures suggested it was more than half.
He said: "This is not an exercise in spin, it is an exercise in honesty. We are saying this is the scale of the challenge."
Sandwell and West Birmingham NHS Trust, which runs City Hospital in Birmingham, reported 2,101 patients had been waiting more than 13 weeks for tests - around a quarter of all those currently waiting.
University Hospital Birmingham, which runs the Queen Elizabeth Hospital, has 1,881 patients who have been waiting more than 13 weeks, more than a quarter of the total.
University Hospital Coventry, which runs Walsgrave Hospital, reported 3,366 patients had been waiting more than 13 weeks, around a third of the total.
And 6,902 patients - around half - have been waiting more than 13 weeks at University Hospital of North Stafford-shire, which runs hospitals in Stoke, around half of the total.
A spokesman for the Staffordshire Hospitals said: "We recognise that improving the waiting times for diagnostics is an important part of reducing overall waiting times.
"We took it very seriously even before the Government published the figures. We are making progress, but there is more to do."
Tim Atack, hospital director of City Hospital in Birmingham, said: "The Trust has a waiting list group which meets regularly to identify ways in which to reduce waits.
"The Trust has very good waiting times in many areas. For example, at the end of March there were no patients waiting more than 13 weeks for an MRI or CT scan, and only one patient who waited eight weeks."
Mr Burnham said he was aware that targets would not be "easy" to meet.
"But in the past, when we have set challenges to the NHS, it has risen to them."
Mr Burnham said there was a wide variation in the performance of trusts.