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He has left an indelible mark on his adopted city: Mackay's practice, Martorell Bohigas Mackay Arquitectes, is credited with much of the planning and architectural design that helped the city win the RIBA's Royal Gold Medal for Architecture in 1999. It has been praised as "perhaps the most interesting example of urban regeneration in the western world today" by Lord Rogers, chairman of the government's urban taskforce.

This is the background that has made Mackay the darling of the British lecture circuit, and has secured his involvement in five large urban regeneration projects in Britain and Ireland, including the the Leaside industrial marshlands in east London. Hastings and Bexhill will be his biggest to date, as it was recently earmarked for a generous £400m budget by the South East England Development Agency.

Mackay's connection with Barcelona began while at college, when he met and married a Catalan woman, Roser. Four years after moving back to her home city, he formed a partnership with Josep Martorell and Oriol Bohigas, who had set up their practice 11 years earlier.

Mackay now enjoys dual nationality, and his south-coast burr is mixed with a Spanish staccato. But it is what he says, rather than how he says it, that makes an impression.

He has a facility for expressing himself in a jargon-free way, with illustrations drawn from a wealth of examples carried out by his 30-strong practice. This talent has led to a string of books, articles and visiting professorships. In short, Mackay is a force.

One of the practice's earliest and most famous regeneration programmes was Barcelona's "100 projects" in the early 1980s. These were small-scale improvements to public spaces around the city that were deliberately carried out with no attempt to integrate them into a city-wide development plan.

Speaking at an RIBA conference entitled "Revitalising the European City" last month, Mackay explained: "The programme concentrated on places that would be easy to change. These gave identity to neighbourhoods, so people could begin to feel pride in where they lived."

Public space is the domain of public responsibility, and should not be handed over to the private sector

Mackay's Barcelona experience has given him a dynamic approach to politics at municipal and national levels. Speaking privately after the lecture, he said: "In any urban design or intervention, you must go hand-in-hand with the politicians. It's a combined work." His other oft-repeated tenet is that: "Public space is the domain of public responsibility, and should not be handed over to the private sector."

Turning to his British projects, frustrations emerge in the conversation. The firm's first scheme, for which it was appointed in 1997, was to propose a wide boulevard and tramline link between Cardiff Bay development zone and the city centre. "After our scheme was approved by the council, it went out as a PFI. This meant we no longer had anything to do with finishing the project, which I think is one of the problems in the UK. The author of a job is much more capable of assisting anyone who takes the job on in indicating which things are important and which are not. You can't write down a city project, because cities are always changing, with new problems cropping up."

Mackay's current source of frustration is the Leaside regeneration project in east London, for which the firm was appointed in 1999. Mackay's masterplan is to replace crinkly tin warehouses with housing and upgrade the riverside landscape. He blames central government for lack of funding and legislation. "Land assembly, I would think, is fundamental. If you're going to regenerate the area, there's got to be government money, which will come back in land values."

British architect-turned-developer Roger Zogolovitch has worked with Mackay on the Hastings project, turning his urban design theory into a practical property bottom line. "David has very good strategic ideas, with a broad understanding of infrastructure and development," says Zogolovitch. "He has done extremely beautiful plans that meet all the criteria of the urban taskforce and are practical and viable."

However, as Mackay is discovering, one of the main differences between regeneration in Britain and Barcelona is that in Barcelona, regeneration is planning-led, but in the UK, it is developers that are in the driving seat. "The UK system is entirely reactive, and we suffer the consequences," Zogolovitch says. "This is all very frustrating for David."

Despite these irritations, Mackay remains the eternal optimist about the rebirth of British cities. "In the Victorian and Edwardian ages, public authorities and private patrons took pride and collective responsibility for their cities, with their grand city halls, squares, avenues and lighting standards. I think there is now a movement to recover this responsibility for public space, which has been lost. You don't have to look to America or the Continent for inspiration; it's here already."