Maltese tenor Joseph Calleja has been showered with rave reviews since the opening night of the Verdi opera Simon Boccanegra at London's Royal Opera House last Tuesday, where he is singing alongside the legendary Placido Domingo.

Writing for The Times of London, critic Neil Fisher said: "In the tenor role of Gabriele Adorno, the fabulous Joseph Calleja, a protégé of Domingo, made me think not of his elder co-star but more of a young Pavarotti: in his care, the future of tenor singing looks pretty rosy."

While Erica Jeal of The Guardian said after praising Domingo: "The biggest cheer during the actual performance goes to Joseph Calleja, singing Domingo's old tenor role... with an ardent intensity that caps anything else he has done here."

When this version of Boccanegra was first announced, it was initially met with a degree of trepidation and a little scepticism, because the 69-year-old, one of the world's best ever tenors, decided to sing the leading baritone title role rather than the tenor role he had conquered in the past. The reasons Domingo cited were a life-long wish to do this project - a comment that took on more poignancy after he was diagnosed with colon cancer last March - and, as the sun is beginning to set on his illustrious career, his voice is now better suited to it than ever.

If the opera press were a little doubtful, the paying public certainly were not. When tickets went on sale for this production last March, they sold out within minutes and have since been changing hands on the internet for over £600 a seat. Caroline Bailey, director of marketing at the Royal Opera House, told The Sunday Times: "We've seen a terrific response to tickets going on sale... Sometimes there are landmark 'must see' performances of a lifetime and this will be one of them with Domingo in the title role, Antonio Pappano conducting and Marina Poplavskaya as Amelia, Joseph Calleja as her lover and Ferruccio Furlanetto as Fiesco."

Though Calleja describes it as an "opera-goers opera", he says: "Boccanegra is really very beautiful and subtle although admittedly it does lack the 'star tunes' of let's say a Rigoletto. This didn't diminish the success we had with the audience which was phenomenal... The opening night was special and it's a good thing that London's notoriously difficult critics gave it five stars."

Phrases like "nearly stealing the evening is Joseph Calleja... (who) supplies the most exquisite singing of the evening" (George Hall from The Stage) and "he brought the house down with his aria, and rightly so" (Melanie Eskanazi) are evidence that the Maltese tenor's contribution to those stars is significant.

Rupert Christiansen from The Daily Telegraph underlined how significant: "If one was ruthlessly marking on points, one might judge (Domingo) outclassed vocally by Ferruccio Furlanetto's imposing Fiesco or the gloriously youthful tenor of Joseph Calleja. But if some of Domingo's old power has gone, and the finesse with it, the peerless nobility and sincerity remains."

But Boccanegra has not just been about being a hit with the critics for Calleja. Through the rehearsals, which started in the second week of June, it also reacquainted him with Domingo, whom he had first met during the Spanish tenor's competition for up and coming singers, Operalia, back in 1998. The Maltese tenor is understandably full of respect for the opera giant, whose stamina is the stuff of legend.

"Placido arrived a couple of days late for rehearsals, and during the first rehearsals without him I often wondered how his approach towards this role would be, given his gargantuan experience and megastar status. Once he arrived, he came straight from the airport where he had just landed from LA and I was flabbergasted to realise that he was actually looking forward to rehearse and go through the role with the rest of the cast there. His approach still fresh and unscathed from over 45 years of rehearsing and performing.

"I knew what he could do and his general attitude. This was, however, the first time I watched him rehearse for a fully staged opera. This confirmed to me once again that opera is not a job but more of a vocation, a 'higher calling' if you like. Watching him rehearse with all that enthusiasm and willingness to explore his role further gave me hope that, most probably, my own burgeoning enthusiasm shall stay till my voice leaves me."

Has Calleja felt the need to raise his game in the company of such a name? "I always give my very best. Of course, being in Placido's presence would put anyone under more pressure as he attracts incredible media attention. He is truly very humble and during rehearsals - the breaks, of course - we had long discussions about technique and voice."

But the past few weeks with Domingo have not all been about the serious business of opera. The serious business of football has played a part too, even if the Maltese tenor managed to annoy the grand old man a little on that score. "During the first week of rehearsals he requested that no one would tell him the scores of the ongoing football matches. For some reason this 'request' didn't reach me and I kept telling him. He politely didn't tell me not to. The last straw came when Spain played when he finally said: 'Jose - he pronounces my name in Spanish - please do not tell me the score today as they are recording the game for me."

The Times of London critic, though enthused with the "beauty and power" Domingo's voice still retains, went on to suggest that he should exit the scene while still at the top. However, Calleja does not see things in the same light. "I certainly get the argument of the very knowledgeable Mr Fisher. There is wisdom in pointing out that a superstar should retire at the height of his game. Having said that, I do not really hear any damage or audible decline in Placido's voice when he sings in the baritonal range. If he is happy and healthy I think he should continue doing it - in his stead I certainly would!"

Calleja, just 32, should have some way to go before he gets there. For him, the future is only looking brighter and brighter. Adorno has been his third role debut of this season alone. He is recording a new album for Decca records in Geneva this summer and he will feature on two DVDs to be released very soon - in the title role of Hoffmann which he sang at The Metropolitan in New York last December and his performance in last year's La Traviata at the Royal Opera House alongside American soprano Renée Fleming. He is also expected to make a long awaited debut at La Scala - the only major theatre so far he has not graced.

But for the time being, he is very happy to be in London for the next few weeks, where he will also perform in a concert version of Boccanegra at the Royal Albert Hall (on July 18) as part of the BBC Proms series, before coming to Malta for a grand concert on the Granaries with US star Dionne Warwick and Italian singer Riccardo Cocciante (July 24).

"Even though I was never part of their young artists programme I have to say that the Royal Opera House has nurtured me and given me all the opportunities and I have cherished every minute I get to rehearse with Maestro Antonio Pappano." They are cherishing him too.

Tickets for the July 24 grand concert on the Granaries are available from nngpromotions.com and all Agenda, Vodafone and Exotique outlets. A recording of Simon Boccanegra will be broadcast on the UK channel BBC2 on Saturday at 8.30 p.m. (CET).