Surgeries are paying locum GPs up to £700 a day to cover shortages, it has emerged.
And it has raised concern that a shortage of doctors is resulting in some agencies "exploiting" the situation by charging high amounts.
NHS figures published ahead of clinical commissioning group (CCG) board meetings showed there had been a "sudden decline" in Suffolk and north Essex GPs in the first quarter of the current financial year.
While there was a slight recovery between June and September, the area was left with 446 full-time equivalent GPs excluding trainees, compared to 478 in September 2018.
Dr Dan Poulter, who works as a mental health doctor alongside his duties as Central Suffolk and North Ipswich MP, said there were several reasons behind the decline - including more GPs retiring and others not wanting the stress of full-time work.
He also said controversial changes to NHS pensions means many doctors are choosing to leave the profession early.
However Dr Poulter said there was an "attraction for a lot of younger medics in working as locums" - temporary GPs who cover staff shortfalls - which agencies are, in his view, exploiting.
This newspaper has seen roles advertised in the area for up to £700 a day.
"The agencies are the big beneficiary of this," Dr Poulter said.
"The money is a waste really, because it is going towards a locum agency.
"Clearly, it is not desirable. I'd rather see the money that is going to agencies going to patient care and the NHS.
"Some of these agencies make a very good living out of exploiting staff challenges in the health service. It is not an acceptable state of affairs."
Dr Poulter has encouraged GP surgeries in Suffolk to build their own collective bank of locum doctors, rather than using agencies, in a bid to save money.
Dr John Havard, partner at Saxmundham Health, argued the money charged by some locum agencies was "extortionate", adding: "It's not like we can afford to do that."
He also said locums are not able to provide the same continuity of care.
David Pannell, chief executive of Suffolk GP Federation, added: "The reduction in Suffolk's GP numbers has been expected because a significant portion are now entering retirement age. This trend is likely to continue for several years.
"Many GPs wish to work as locum doctors and the federation supports them to do this.
"Locums are a key part of Suffolk general practice because they can provide cover when regular GPs are on holiday or unwell."
Amanda Lyes, director of corporate services and system infrastructure for NHS Ipswich and East Suffolk, NHS West Suffolk and NHS North East Essex CCGs, pointed out that: "Although we've seen a fall in the number of GPs, there has been a sizeable increase in the number of other health professionals employed by GP practices who are providing direct patient care, such as physician associates and physiotherapists."
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