DeVere Spain, part of the worldwide deVere Group, has formed a partnership with Spain and Portugal-based law firm Martínez-Echevarría.
The move was prompted by a surge in demand for tax advice post Brexit by UK citizens living in the EU, it said in a statement on 2 March.
James Green, deVere's divisional director of Europe, said: "Since Brexit, British citizens can only stay in the Schengen zone, including Spain and Portugal, for 90 out of every 180 days.
"This, amongst other factors, has pushed many expats into obtaining residency. As such, there are more international residents in Spain and Portugal seeking tax advice than ever before. The jump in demand in the last 12 months has been considerable.
"In addition to Spain/Portugal-orientated advice, expats, whether they're retired, employed or self-employed, have specialist cross-border tax needs as they often have financial ties ‘back home' too.
"Therefore, we're delighted to have formed an alliance with major legal practice Martínez Echevarría, who will offer our clients tax advice, using their expertise and experience to ensure optimal outcomes."
Fabrizio Trabison, deVere compliance office and deVere Spain director, said: "This service adds a huge amount of value to all our clients.
"Besides taxation, amongst Martínez-Echevarría's specialist areas of knowledge are inheritance and family law, labour law and social security, real estate, corporate and commercial contracting and corporate governance.
"We expect take-up of the service to be extremely high from our clients."
Eugenio Martínez-Echevarría, partner of the law firm, said: "We are very happy to be forging this top tier strategic alliance with deVere, a highly respected organisation that, like us, has a client-centric focus.
"We're confident that their size, scope and infrastructure capabilities combined with our legal and tax expertise is a powerful, valuable combination for deVere clients in Spain and Portugal."
All clients of deVere Spain and deVere Portugal will be able to make tax enquiries through their advisers with immediate effect.