In the poll of nearly 2400 travellers, by website TripAdvisor, the British capital was voted the dirtiest in Europe, home to the worst-dressed people and the most expensive.
Paris did not come off any better, with travellers saying it was the least friendly city and the second-most expensive.
But both popular destinations redeemed themselves in the online poll, with Paris voted as having the best cuisine and best-dressed people, while London was seen as having the best nightlife, best public parks and most free attractions.
"Europe's capital cities all have their highs and lows, but no other continent offers travellers' such a wealth of culture and sights within such short distances," TripAdvisor spokesman Luke Fredberg said in a statement.
"Despite London emerging as the dirtiest and most expensive city, its fantastic free attractions prove that you don't need to be a millionaire in order to enjoy the capital."
Venice beat both Paris and Rome as Europe's most romantic destination, but it was also voted the third-most expensive, after Paris and London.
Travellers seeking friendly locals should go to Dublin, while the health-conscious may prefer Denmark's Copenhagen, voted the Europe's cleanest city.
Prague was picked as the best bargain destination, while people seeking visual treats should head for Spain's Barcelona, but avoid Warsaw in Poland, which were seen as the cities with the best and worst architecture respectively.
And the most boring city? Travellers voted for the Belgian capital Brussels, with Switzerland's Zurich a close-runner up.