Senior doctors have warned that a major shake-up of sexual health services in three London boroughs will lead to more unintended pregnancies and sexually transmitted infections.

As concerns grow about the funding of sexual health services across England, 14 experts in the field at St George’s hospital in south-west London have written to NHS and local council leaders branding the money-saving changes “unsafe, unworkable and unsustainable”.

In the letter, obtained by the Guardian, they say the Central London community healthcare NHS trust’s plans for running the services from 1 October are not clinically safe.

Plans to shed 30% of staff and close a long-established clinic as part of a switch in providers will mean patients with STIs are denied specialist counselling and undermine arrangements for safeguarding young people, they say.

The row follows growing concern among medical organisations about a series of sexual health clinics across England either shutting or limiting access as councils cut spending after ministerial raids on their public health budgets that have created a “perfect storm” in sexual health.

The Local Government Association said this month that patients could face longer waits for STI testing and contraception because services had reached a tipping point.

A dispute has broken out over how Central London community healthcare (CLCH) plans to provide sexual health services in Wandsworth, Richmond and Merton when it replaces St George’s as the main provider. It has still not identified permanent alternative venues for services it will run with far fewer staff.

Dr Paul Lister, a sexual health consultant who was head of the St George’s service until he retired in June, said the changes would lead to the potential implosion of sexual health services.

In their letter, the 14 co-signatories say: “We are writing to express our serious concern regarding the staffing and service models proposed by CLCH for the sexual health hub in Wandsworth and highlight a need for an immediate review of approach. We are of the opinion that the current proposals are unsafe, unworkable and unsustainable.”

They say patients will receive worse service when the main facility in Wandsworth, the Courtyard clinic at St George’s where staff see 29,000 patients a year, closes as part of the transfer. The service specification says there should be a single clinic, but CLCH is proposing to use two, neither of which have been identified, they say.

Of the existing service’s 68 staff, 24 are expected to lose their jobs, including nurses and sexual health advisers who counsel victims of sexual assault and child exploitation. CLCH will not be able to staff its new service properly for the Monday to Saturday opening hours it has promised, the experts say.

“While we appreciate the austere financial climate and the need for change, we do not assess the current proposals by CLCH to be clinically safe,” they add.

Lister said: “The plans for new premises and staffing levels with many redundancies mean the service is undeliverable. The reduced access to a service of considerably reduced quality will result in higher rates of STI, HIV and teenage pregnancy.”

CLCH hopes to provide effective services by encouraging as many as 30% of patients not to attend a genito-urinary medicine clinic but instead to test themselves for STIs using a free kit ordered online. Rather than seeing patients face-to-face, it will run the service using “self-care and self-sampling that delivers a cost-effective and user-friendly service that is high quality and clinically driven through nursing and medical input”, CLCH said.

“The service we have been commissioned to deliver is different from the current one and will mean changes for sexual health staff. We are working hard to support staff through these changes. We are confident that we will deliver a high quality service in line with the new London-wide approach to sexual health services. Residents in Wandsworth, Merton and Richmond will be able to get help from a range of locations six-days a week; with on-line booking and self-service check-in that offers convenience and confidentiality.”