Perrott Phillips crosses four frontiers and drops anchor in the centre of three capital cities on a riverboat cruise along the Danube.

The American tourist was aghast. It was 8am and the German passengers aboard the MV Mozart were clinking glasses of sparkling Prosecco wine and muttering ''Guten appetit!".

''You people drink wine for BREAKFAST?", he exclaimed unbelievingly as he walked away from the mountainous buffet nursing a glass of mineral water.

Some 24 hours later, the Americans had bigger problems on their mind. ''How many Hungarian forints to the dollar?" they wanted to know. ''And can you change German euros into Austrian euros and then back into Slovakian koruna?".

We were sailing up the Danube on a 720-mile round trip on the biggest riverboat in Europe. The MV Mozart is 393ft long and 75ft wide and sitting on the green-carpeted sundeck is like sunbathing in the middle of a football pitch.

In seven days - in which the Danube changed to the Donau, then the Duna and finally the Dunav - we passed through eleven locks, crossed four frontiers and dropped anchor in the centre of three capital cities.

We set sail from Passau, where our Hungarian-born captain had to reverse in midstream. No ten-point turn for him. He spun the Mozart round like a top. ''Big as she is", he said, ''I can turn the Mozart on a euro.''

Napoleon called Passau ''the most beautiful town in Germany", but he didn't trust the townsfolk. At his military HQ in Theriesenstrasse, he ate only fried eggs in case someone tried to poison him.

Colourful buildings, like children's building blocks, line the towpath, pierced by the spires and domes of a cathedral and countless churches. Cafe tables spread below the walls of the riverside Town Hall, decorated with paintings of knights in armour.

What looked like a solid wall of pine forest closed in on us on either side as we nosed through the Aggstein narrows. The cliffs rose to 1,300ft, where a 12th century castle perched on a crag like a lost eagle.

We skirted willow-covered islands and slid past the combed and terraced vineyards of the Wachau. Tiny villages topped by churches with domes like pickled onions were dotted among fields of tobacco and maize and orchards of plums and peaches. Beaches the size of a Kleenex appeared between the conifers, often with a solitary nudist spread-eagled on the sand.

Durnstein's famous blue and white abbey tower, like a Delft pepperpot, was dwarfed by the ruins of the 12th century castle where Richard the Lionheart was imprisoned during the Crusades. He was only discovered when his loyal minstrel, Blondel, strummed his favourite song outside the walls. An event celebrated by every souvenir shop in the village.

The wonderful thing about the Mozart was that it could nose right in to the centre of great cities, rather like sailing into Piccadilly Circus.

In Budapest, it was barely a step into Vorosmarty Square, where I sat at the venerable Gerbaud Cafe - its counter piled with 100 different cakes - and ordered from a menu that hadn't changed since the days of the Austro-Hungarian Empire. A goose-liver sandwich, an Esterhazy Chocolate Bomb and a glass of Imperial Tokay cost only £7.

Saucy-looking girls in colourful national dress beckoned me into souvenir shops as I walked down Vaci Ulica, lined with designer boutiques and smart cafes. At the National Museum, I was handed a pair of baggy carpet slippers to pad silently round the cases containing Hungary's royal regalia.

In its 1,000 years, the jewel-studded royal crown has been battered, buried, stolen, lost, put up for sale - the deal was clinched in Trafalgar Square, London - seized and hidden in Fort Knox before finally returning to Hungary in 1971, its cross bent to one side.

The Mozart's waiters were all Hungarian, as dashing as Hussars and oozing charm by the waggonload. One greeted women passengers with the old-fashioned compliment, ''Kezet czokolom'' - ''I kiss your hand, fair lady". The women almost fainted with delight.

The Mozart is 5-star rated and service is impeccable. Dinner menus spread across seven courses and even the breakfast buffet includes smoked salmon and halibut, eight cold meats, five cheeses, four pates and 15 different breads. Not to speak of the Prosecco.

It's more like a floating country house hotel than a riverboat, with spacious lounges, indoor swimming pool, library, fitness centre, sauna and hairdresser. There are only 200 passengers, so cabins are roomy, with picture-windows, TV, fridge and phone.

We sailed into Vienna to find every other person dressed like Mozart or his sister. They turned out to be advertising rather pricey evening concerts of Baroque music in 18th century dress. Strictly for the tourists.

But I saw von Karajan conduct La Boheme and watched Callas singing her heart out, all for nothing. Yes, I know Karajan and Callas are dead. It was a movie. Vienna's biggest bargains are the open-air filmed concerts and opera shown every summer evening in the Rathausplatz and featuring some of the world's greatest artists.

There's free seating for 2,000 in front of the enormous screen. Round the edge, takeaway stalls sell everything from Spanish paella to Greek souvlaki - there was even a Chinese ''Nudelmeister'' drawing out noodles like a skein of wool - with beer, wine and cocktails on tap.

An evening of absolute magic for the price of a beer and a sausage.

The Slovakian capital of Bratislava was another revelation - unspoiled, uncommercialised, empty of traffic and full of friendly people.

Streets of 17th century houses lead beneath arched gateways to lovely old squares with fountains shaded by plane trees. Traditional shops sold Slovak glassware, country pottery and the kind of wooden toys I thought had vanished in the 1930s and now only reappear on the Antiques Roadshow.

''Only £5 for an hour's tour round the Old Town", said the driver of a horse-drawn fiacre. We jangled by the house where the composer Hummel was born in 1778, past the 14th century cathedral where 11 Hungarian kings were crowned and the Primate's Palace, with its collection of rare 17th century tapestries, woven in Surrey.

At breakfast on the last morning, the abstemious American brushed into me carrying a glass of Prosecco from the buffet.

''You know", he said, ''I could get used to this wine for breakfast idea.'' He paused a moment, then added defiantly, ''Way to go!". I knew exactly how he felt.

TRAVELFACTS

* Perrott Phillips was a guest of Peter Deilmann Cruises, Suite 404, Albany House, 324/326 Regent Street, London W1B 3BL (Tel, 0207-436-2931).

* Seven night Danube cruises start at £828 for an inside cabin (£1162 for river-view cabins), including return Lufthansa Heathrow-Munich flights, transfers, all meals. Regional supplements will be charged for those flying from Manchester or Birmingham, if flights are available.