After years of controversy over the issue — which included a split statutory report which was ultimately shelved, a government- ordered review and threats of legal action — it is understood that a compromise boundary extension has now been agreed.

Details of the new boundary line, which will result in the city’s population increasing by almost 100,000 to about 220,000, and which will shape development of the city for the next half century, is set to be unveiled to both city and county councillors today.

A source described today as a “make it or break it” point in the long boundary extension process.

It is understood the compromise extension proposes that Ballincollig, Blarney, Glanmire and Cork Airport become part of the city, as proposed by the Mackinnon review group.

However, the commercial rates-rich area of Little Island and Carrigtwohill, east of Dunkettle, which were part of the Mackinnon report, are expected to remain part of the county under the compromise deal.

It follows weeks of intense negotiation between senior city and county council officials, overseen by an implementation oversight group which was set up to implement the boundary extension as recommended by the Mackinnon review group.

Mackinnon was set up to review the shelved 2015 Smiddy report which recommended the effective merger of the city and county councils and the creation of a super-council.

However, Mackinnon recommended the retention of the two local authorities and a significant city boundary extension.

The chiefs executive of both city and county councils will be briefed on the proposed boundary during what could be the final meeting of the Mackinnon implementation oversight group this morning.

Both chiefs executive will then update their councillors on the outcome of the oversight group process at special local authority meetings which have been called for this afternoon.

Councillors will be asked to endorse the proposed boundary extension which the oversight group proposes to submit to the Local Government Minister Eoghan Murphy.

Last week, Tánaiste Simon Coveney had warned both local authorities to agree an extension or face the prospect of government imposing an extension.

The city’s boundary was last extended in 1965.

Cork City Council has been arguing for an extension for the last 20-years to give it enough land and population to grow, and act as a development counterweight to Dublin.

Cork County Council has argued that a significant boundary extension will result in a significant loss of its rates income and affect its ability to deliver services.

Details of a compensation package which will see the city paying the county for any loss in rates income arising from an extension are also expected to be part of the extension proposal.