Letters have been handed out to Manpower agency workers at the site announcing the scrapping of jobs on the XF and XJ lines, despite plans for a new £320 million aluminium body shop.
Jaguar Land Rover refused to confirm the figures, but the Post understands it may be as many as 700 people.
The investment marked the largest single cash outlay at Castle Bromwich in its history but it is not enough to save the jobs of agency workers hoping for regular contract work.
The letter handed out by Manpower to agency staff states: "Following a significant period of intense launch activity at Castle Bromwich, Jaguar Land Rover has informed Manpower that the company is now to introduce a normalised production schedule to support its build plan for the coming year in line with normal industry practice.
"We are therefore writing to you to advise that unfortunately your Manpower assignment at Castle Bromwich is likely to end before you become eligible for a short-term contract with Jaguar Land Rover.
"Based on current plans, the majority of Manpower employees at Jaguar Land Rover will be reassigned from the assignment at the end of May and October 2015."
Jaguar Land Rover spokesman Ken McConomy said: "These people were brought in over the last 12 months to help with the launch of the new XF, the new refreshed XJ and the F-Type.
"They were brought in on the premise that they were required for a short time. This is normal industry practice."
He was unable to confirm reports that between 400 and 700 agency staff had been handed letters.
"There will be some agency staff left but I do not know the numbers," he said.
But a shopfloor source said: "The Manpower official has been handing out letters to agency staff. They are being stood down although there are opportunities to go to the new engine plant at Wolverhampton.
"It is a great shame after all the positive news of the last few months. There are a lot of people saying they can't wait to get into Jaguar Land Rover and it can't be easy for them after this.
"I really do think that somebody somewhere has over-estimated the manning levels. There are hundreds of people walking around on quality tasks."
The source said the F-Type lines at Castle Bromwich had been reduced to one shift after two and a half years of two shifts, with morning and afternoon working.
"They have been put on normal day-shifts from 6.30am to 3pm. For the last two and a half years there have been 6.30am to 2.30pm and 2.30pm to 10.30pm shifts.
"They are saving money by not paying a shift premium. People are losing out to the tune of £100 a week."
Mr McConomy confirmed that vacancies were available at the Wolverhampton engine plant but could not comment on reports of the shift-reductions on the F-Type lines.
JLR has created thousands of new jobs at Solihull, Castle Bromwich and Halewood over the last four years, with a string of new models following huge investment from parent group Tata.
In its latest investment announcement last month, JLR revealed it was spending £450 million at Castle Bromwich to produce the next generation Jaguar XF aluminium car.
Sales of Jaguars built at Castle Bromwich are almost double that of five years ago on the back of international demand for the XJ and F-Type.