National World, owner of The Scotsman and The Yorkshire Post, has been drawn into a dispute over invoicing after its largest shareholder tabled a potential £56.2 million takeover.

Irish publisher and direct marketing company Media Concierge, which owns 27.8% of National World, went public on Friday with an offer that it had presented to the company’s board on 31 October.

National World responded by saying it was investigating “a potentially systemic pattern of historical invoicing irregularities in relation to the activities of entities affiliated with Media Concierge”.

These related to an advertising sales deal between the two companies. It said that entities affiliated with Media Concierge were also withholding revenues of £4.4m.

Media Concierge, which owns a number of newspapers in Ireland and runs direct mail and advertising operations, dismissed the allegations as “completely baseless”. It said National World had provided no evidence to support its claims, noting that they coincided with the takeover approach.

The spat unfolded in statements to the stock exchange after Media Concierge offered 21p a share for National World, a 40% premium to its closing price on Thursday. Shares in National World jumped 4p (26.67%) to 19p.

National World said it has undertaken a significant amount of work with the company’s financial and legal advisers to evaluate the possible offer.

It said the board has confidence in its strategy for value creation as an independent business “but acknowledges its [the offer’s] potential merits”.

National World was set up as a media investment vehicle in 2020 by former Mirror Group chief executive David Montgomery.

Later that year, it acquired the regional newspaper publisher JPI Media for £10.2 million, taking control of dozens of titles that include the The Newsletter in Belfast, the Sheffield Star and dozens of local titles such as the Arbroath Herald, the Stornoway Gazette and Falkirk Herald.

Sources say Media Concierge had become dissatisfied with Mr Montgomery’s leadership and not supported him at the AGM in May. It felt margins had not improved sufficiently as the company has engaged in a series of lay-offs.

National World is the underbidder for the Telegraph titles but Media Concierge is thought not to support the move.

Media Concierge has until 5pm on 20 December to announce a formal intention to make an offer or withdraw.