The hospital has admitted it is being paid to house the transmitter

Elin Jones, who represents Ceredigion, said there was a question over the safety of such masts.

But Ceredigion and Mid Wales NHS Trust said radiation levels complied with national guidelines.

The trust added that it was being paid for housing the technology, but would not reveal the amount.

In recent years, there have been protests throughout the UK about the positioning of mobile phone masts near homes, amid worries that they affect people's health.

There's still a question over the safety of these masts Elin Jones AM

The transmitter's owners, Hutchison 3G, said it recognised people's concerns, but it had consulted the community and received no objections.

It is unclear when the transmitter will be installed.

Ms Jones said it would be just 50m from some homes.

"We all know when we go into a hospital we have to switch off our mobile phones because they could affect medical equipment, yet the trust is placing a powerful mast on the hospital's roof," she said.

"There's still a question over the safety of these masts. I think we should take the precaution of not having them where there's a possibility of them having an impact."

Ms Jones also said she had received complaints from some residents.

Elin Jones said the transmitter would be 50m from some homes

But Ceredigion and Mid Wales NHS Trust said it agreed to house the transmitter after assurances there had been no local objections.

A spokesman added the trust had also sought assurances that it would not interfere with medical equipment and complied with recognised radio frequency public exposure guidelines.

Hutchison 3G said it held talks with Ceredigion Council last year.

"It was agreed with the council that Bronglais hospital would be the best environmental solution", said company spokeswoman Verity Stanford.

"It is ideally located to provide third generation mobile phone coverage to east Aberystwyth."

Ms Stanford added it was in line with government planning policy which encourages operators to use existing structures to put the masts on, and that the company had received no objections.