Andy Furey is the National Officer for Communication Workers Union, which represents staff working in Crown Post Offices.
He told ITV News: "A Welsh town like Holywell needs a Crown Post Office. If we lose it, this could be the death knell for the high street in Holywell."
Delyn MP David Hanson has told ITV News more detail is needed regarding proposals to franchise Holywell's Crown Post Office.
"Part of tonight's meeting is to hear from local people as to what they think of the service," he added.
The Post Office says its 370-branch crown network is losing £37million a year, which cannot be sustained.
The Post Office is committed to keeping branches on high streets across the UK. We know from decades of experience that the best way to do this for 70 of our Crown branches is to partner with a suitable retailer. We have announced our plans to seek a partner with whom to operate our Holywell branch.
– Post Office spokesperson
In Wales we are seeking partners to help run our branches in Holyhead, Llangefni, Holywell, Rhyl, Port Talbot, Neath, Morriston (Swansea) and Carmarthen. There are currently 20 Crown Post Offices in Wales out of a network of over 950 branches, therefore around 98% of the total network in Wales already operate successfully through franchises and agreements with retailers of this type.
Delyn MP David Hanson will be among the speakers at this evening's meeting. He says the future of Wales' crown post offices needs to be protected.
I'm concerned about the impact the changes to the Crown Post Office might have for its staff and the local community it serves in Holywell, as post offices are often vital for local people.
People across North Wales really value their Post Offices and want to be able to access a range of services like motor vehicle licences, passport check and send services, bureau de change and to receiving appropriate advice and assistance for these documents.
– David Hanson MP - Labour, Delyn
So the announcement of changes by the Post Office raises the real prospect that these services may no longer be easily available to local people.
The UK government claim to have no planned closure programme, but this would seem somewhat like closure and reduction of services by stealth, so the government must answer how the future of the 20 Crown Post Offices branches across Wales are to be protected.
– Jenny Windsor, Communication Workers Union
Holywell High Street has been struggling in recent years and the Post Office is a key attraction which draws people into the town centre. With superstores such as Tesco on the outskirts of the town, if the Post Office was to be lost from this location other local retailers would lose out.
There are ways to keep this office open and we're calling on the company to listen to the views of its staff and customers. We're urging as many local residents as possible to come along to this meeting to have a say in the future of their local services.
Campaigners against plans to franchise Holywell's Crown Post Office will hold a public meeting later.
The High Street branch is one of eight being considered for franchise out of 20 Crown Post Offices across Wales - leaving 12 by 2015.
The Post Office said it is committed to keeping branches on high streets across the UK, pointing out that around 98% of the total network in Wales already operate successfully through franchises and agreements with retailers.
But Communication Workers Union, the trade union representing staff working in Crown Post Offices, says Post Offices are 'key attractions' in town centres and believes there are ways to keep Holywell's open.
A petition has also been signed by thousands of people objecting to the plans.
The meeting will be held at Holywell Leisure Centre from 6pm.