A controversial investigation by the Revenue into whether thousands of older people have undeclared tax liabilities may draw to a close shortly on the basis that it does not make economic sense to continue it.

About 150,000 pensioners received letters from the tax authority in late 2011 because they may have owed unpaid taxes. Many did not have any liability however.

Investigations by the Revenue to date have yielded €11.5 million based on detailed examination of about 6,000 cases. Only 2,900 were found to have a liability. The average yield per case was about €4,000.

Officials have been focusing on pensioners who had incomes of between €40,000 and €50,000 and who had not declared their State pension to the Revenue.

“We are currently reviewing the progress on the project to date and evaluating the effectiveness of the programme, taking into account the level of resources that has been engaged in the work,” a document states.

“This will help inform us before we decide whether we will pursue cases below €30,000 as the tax at risk involved and the potential outcomes from those cases . . . may not represent the best use of our PAYE compliance resources.”

This probe of pensioners followed a data-sharing exercise in which the Department of Social Protection sent files to Revenue on pensioners in receipt of State pensions. worth €4.5 billion.

Revenue calculated that €300 million of this was additional income and had not been declared for tax purposes, and issued letters to 150,000 pensioners to make them aware of potential liabilities.

The move sparked political controversy and the Revenue ended up fielding thousands of phone calls from older people who were worried about their pension liabilities. Of the 150,000 cases, 115,000 are thought to have had some liability. It has also emerged that thousands of pensioners had paid too much tax. Since the mailshot in late 2011, almost 6,000 pensioners have received refunds worth €1.1 million.