The community alarm service for older residents in Glasgow is wonderful, according to Sheena Glass.

"If you fall, you press a button and within half an hour two people are there to help. It's a bit of security," she explains.

However Ms Glass, the chief executive of Glasgow Old People's Welfare Association, said there has been widespread anger at a move by the City Council to charge for the service, which until now has been free. Users were written to in October to inform them of the change. "People are angry, to get a letter one week, expecting you to pay within a few weeks," she said.

Around 13,700 users of the service, provided on Glasgow City Council's behalf by the arms-length company Cordia, have already begun to be charged for the service and will receive bills in the next week, charging them £3 a week.

Ms Glass added: "It's not that people don't want to pay, but there was no consultation. The council weren't even good enough to talk to people and say this is what we are going to be doing."

However there is a growing political row over the council's decision, including the way it was carried out, but with big questions too about the affordability for a group of citizens who, by the very nature of the service, have already been deemed vulnerable.

It is not clear how many users are choosing to surrender their alarms. In its business plan for this year, which The Herald has seen, Cordia discusses charging for the alarm response service. It concedes: "any imposed standard charge for this service - may have a significant impact on the number of service users who will wish to continue using this service".

Other councils have seen the number of service users drop after introducing charges for similar services, but Glasgow City Council says it has not asked any of them for details about numbers.

Under a new scheme people over 80 in the city are entitled to an affordable warmth payment of £100, critics point out, but any of those who use the alarm service will see that benefit eliminated by an annual bill of £156 for their alarm.

SNP Councillors say they have been told up to 3000, around one fifth of those who use the alarms, are deciding instead that they can do without. David Taylor, councillor for the city's Baillieston ward, said he had been given this figure at a recent meeting of his local residents' association. Other estimates are lower, but more people are expected to return their devices once invoices start to arrive through letterboxes in the run-up to Christmas.

There is also unhappiness about the way the decision was taken and communicated. Glasgow City Council insists that it approved the charges in April 2009, and subsequently held off setting a rate as long as possible - which it did in March 2011. Some 24 of Scotland's 32 councils now charge for telecare alarm services, a spokesman added.

However opposition councillors argue that this is far from transparent and most current councillors were unaware that the charges were being introduced.

Nearly half – 38 out of 79 members – of Glasgow's current council were not elected three and a half years ago when the decision was taken.

SNP social care spokeswoman Susan Aitken said: "The way this has been done may not be technically against the rules but it is not an acceptable way to do business.

"Community alarms are becoming more and more central because of policies which aim to keep people in their own homes. That is a good thing. But, even though it is quite clear there is an impact on elderly and vulnerable people the council haven't done any proper calculation of the impact of charging or an equality impact assessment, or looked at how the drop-out rate can be minimised."

Ms Aitken added that there were questions to be asked about the sums involved.

Cordia's business plan budgets income for the Alarm Response Service at £1.787 million annually. But assuming most users do pay bills of £156 a year, that sum would be considerably increased to more than £2m a year. "I would like to know more about the level of charging and whether the council or Cordia are looking to make income from this vulnerable group," she said.

"The fact is that people were panicked when these letters went out and the majority of councillors were unaware it was happening. I'm a new councillor myself and I think the fact that there has been no scrutiny of this by the current council is ridiculous."

Others have also expressed concern about the move. Jim Harvey is director of the Glasgow and West of Scotland Forum of Housing Associations (GWSF), which represents 47 of the 50 housing associations and housing co-ops in the city.

He told The Herald GWSF members had "strong concerns" about the introduction of charges and the consequences for older tenants, particularly those on low incomes.

GWSF is also unhappy about the lack of any prior discussion with housing providers about the introduction of the charge, or how the introduction of charging should be communicated to tenants.

Mr Harvey said he was also concerned about the charges being introduced at a time when pensioners were faced with rising bills for food, heating and other essentials.

"The alarms service provided by the council is an essential lifeline – not an optional extra – for those who have them," he said.

"GWSF members are concerned that some of their tenants may withdraw from the services as a result of charges being introduced."

He also questions the cost-effectiveness of the move, adding: "This will be harmful to individuals, but it will also result in much greater costs to hard-pressed health and social care services."

A spokesman for Glasgow City Council said charges had been introduced in response to ongoing financial pressures. "We believe that £3 per week is a modest sum in return for the reassurance that people can remain safe and independent while living at home." he said. "Those who feel they may struggle to pay the charge may be eligible for financial assistance."

He said the fees for the service will go to the council itself, not Cordia, and added: "The decision to introduce charges for community alarm services was made and subsequently re-examined in accordance with the council's committee system."

Drop-out rates will be monitored as charges are implemented, he said, but the council believes the numbers giving up the service are being exaggerated.

This is a verdict Sheena Glass agrees with. However she remains unhappy. "I don't think many alarms will be handed back. But they should have told people through the media, at the very least," she adds.

"Just sending a letter out of the blue: it's a wee bit cold."