They are calling it Europe’s most advanced Wi-Fi network and trumpeting its ability to allow constant connection to the internet no matter where you are. But tomorrow morning, when the system goes live for the first time, bankers, lawyers and stockbrokers working in the capital’s financial hub will be asking themselves one question: will it work?

Some 350,000 professionals working in the Square Mile have been promised that they will be able to take their broadband with them as they go about their working day. The last thing you want is your laptop cutting out on you when you’re about to close a multi-million-pound deal, so we decided to test the claim and the network with a high-speed taxi journey across the Square Mile — all the while staying connected.

After hailing my cabbie outside the Lloyd’s building, home to the insurance market, I settled in the back and switched on my laptop. My computer had no problem detecting the network and connecting to it. After typing in a user name and password (given to me by the Cloud, the company behind the scheme — normally you would need to pay £4.50 an hour or £12 a month) I was in.

As the taxi headed on a tour of central London’s tourist attractions I selected a YouTube video clip. Fast-moving objects such as rally cars speeding round a track are always a good test of any connection, and the clip of an Audi quattro Sport was delivered seamlessly to my screen with no jumping or cutting out.

Next, as I directed the driver towards the Thames, I tried to stream a music file from MySpace. The Wi-Fi zone encompasses Britain’s most expensive chunk of commercial property, stretching from the Royal Courts of Justice in the west of the City to the Tower of London in the east, and from the Barbican in the north to the Thames in the south.

As we neared London Bridge, the staccato guitar licks of Brianstorm — Arctic Monkeys’ new single — started to sound jittery as the signal weakened significantly. And as we crossed the water the sound was cut dead as if someone had unplugged the guitars. Then my connection was lost altogether.

After turning round and reconnecting, I attempted to make a phone call over the internet using Skype — the internet-based phone service that promises free calls to other Skype users anywhere in the world. Although the call went through easily enough, the sound quality was even more echoey than is usually the case with Skype. Furthermore, as my cab neared the northern limits of the network the voice at the other end cut out several times before the connection was lost altogether, even though we were still well inside the zone.

And therein lies the problem. The Cloud claims that 95% of pedestrian areas in the Square Mile are covered by the Wi-Fi network, which consists of 127 transmitters or “nodes” placed on lampposts and buildings. However, there are blackspots, and if you’re trying to download data or hold a telephone conversation, that is a downer.

Niall Murphy, the company’s co-founder and chief strategy officer, accepts this, saying: “We want to achieve consistent coverage, but fully expect it not to work all the time.”

Partly this is because glass buildings can affect the strength of the signal around them, and that’s something I found out as I tried in vain to log on while standing next to the colossal cigar-shaped building known as the Gherkin that’s almost entirely covered in glass.

But more significantly, the nature of London’s streets inhibits the network’s effectiveness: the City is full of winding roads and small alleys, where a signal simply cannot reach. Medieval town planners clearly did not have Wi-Fi in mind.

Still, think of this new hotspot as like the early mobile-phone networks: patchy, crackly and sometimes infuriating.

Despite which, you know the future has arrived.

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