Most hospitals in England have still to implement the new NHS contract for consultants, according to the British Medical Association.
The contract, which was agreed last year, was supposed to be in place in February.
But the BMA says managers at many trusts are refusing to agree job plans with consultants.
Ministers hope the contract, designed to allow doctors to work more flexibly, will help cut NHS waiting times.
The BMA says the contract needs to be introduced as soon as possible.
"This needs to be done in a short time frame," said Dr Jonathan Fielden, deputy chairman of its consultants committee.
"We have the working time directive coming in for junior doctors in August and their hours will come down dramatically.
"If you haven't resolved the issue for senior doctors, then you won't be able to resolve the issue for juniors."
'Best case scenario'
According to the BMA, just one in four trusts has implemented the new contract although some are expected to agree job plans with consultants over the next two weeks.
"Less than a quarter of consultants signed up and ready to go on the contract," said Dr Fielden.
"The best case scenario we feel for the end of this month is half."
Dr Fielden said the delays were down to some trust managers acting unreasonably and failing to work in partnership with doctors.
"You have to have a partnership and cooperative approach," he said.
"Trusts that are doing that are getting results."
Health Minister John Hutton insisted progress is being made.
"The government agrees with the BMA and is committed to implementing the consultant contract as quickly as possible.
"We have been pleased to see a significant amount of progress has been made by employing organisations in implementing the contract recently.
"We expect that this progress will continue and the great majority of consultants will receive final proposed job plan offers by the end of this month."
But Shadow Health Secretary Andrew Lansley criticised the government.
"The incompetence of this government is shocking. They continue to introduce changes but do not provide the necessary support to ensure that the NHS can carry them out."