The move will see SEC’s international HQ move to Newington’s new London office in Southwark, and bring SEC’s turnover above £22m (€26m).
The deal involves a completion cash payment of £1.2m and a further cash payment of £175,000, and two further earnings-based payments to be made later. SEC may be required by Newington's minority shareholders to purchase further shares five years after completion of the deal.
The Italian company said the deal was part of its plans to expand in Europe in addition to expansion into the Americas and Asia.
Newington CEO Mark Glover indicated that his agency would retain some autonomy, describing SEC as "a partnership of independently-run consultancies".
Naomi Harris will remain MD of Newington’s public affairs division, working alongside Glover and agency MD Briscoe, who will both join SEC’s management committee.
Glover said: "This partnership will provide Newington with the necessary scale to continue to build our integrated public affairs and corporate comms offer at a local, regional, national, European and international level.
"We will retain the values, culture and ethos that have been the foundation of our success to date.
"This is a new business model – a partnership of independently run consultancies which are encouraged to retain the unique qualities that set them apart from the competition."
Newington employs more than 45 consultants in four UK offices. In 2015 the agency, then called Bellenden, generated revenue of £3.1m, up 24 per cent in one year. It works in sectors including education, energy and environment, financial services, health, housing, justice and legal, plus property development and transport.
Fiorenzo Tagliabue, SEC Group’s founder and CEO, said: "We had wanted to invest in a British consultancy that combined a strong track record in the sector with a forward-looking approach when it comes to developing new integrated campaigning techniques, and that is on the front foot when it comes to transparency and ethics.
"Newington met this criteria and we’re delighted it has joined our developing partnership across Europe, the US and Asia."
Speaking in July as SEC listed on AIM, London’s secondary market, Tagliabue said: "Our project is very clear – to build a global partnership that acts as a hub of entrepreneurs who focus their activities and strategies on the business and reputation of their clients."
SEC was the first company to float in the UK since the EU Referendum. Tagliabue said at the time: "I feel certain that in spite of Brexit, this country even in its specific insular condition fully belongs to the community of Europeans because it grows from the same cultural roots and identity, and this is a fact that no referendum could erase."
Announcing the name change to Newington Communications in August, Glover said it was an attempt to convey a "cross-country footprint", with the place name found throughout the UK.