London Mayor Boris Johnson officially welcomed the introduction of the buses at an event at The Mansion House here, bringing the total number of buses in service to six.
The first two electric buses in London have been operating since last December with two more expected to enter service later this year.
The buses, which offer much lower noise and vibration levels compared to diesel vehicles, demonstrate Transport for London's (TfL) commitment to trialling emission-free technology.
The introduction of the pure electric buses is the first step of the Mayor's plan for all single deck buses in central London to be zero emission at tailpipe and make it world's first Ultra Low Emission Zone.
The buses have zero tail-pipe emissions, resulting in lower carbon emissions and improved air quality.
The bus trials will be used to establish whether the technology can withstand the rigours of operating in an intense urban environment like London.
Gopichand Hinduja, Co-Chairman of the Hinduja Group said: "Optare's MetroCity electric vehicle is a prime example of the UK's manufacturing excellence technology and innovation coming together to provide a zero emissions solution to improve air quality in London.
"We look forward to seeing more of these vehicles operating in London and cities across the UK and Europe in the future," Hinduja said on Tuesday evening at a launch event.
"We will be closely monitoring the results of the trials, which may help us adopt this new clean technology more widely in London," Mike Weston, TfL's Director of Buses said.
The buses take around five hours to fully charge overnight and have a range of up to 100 miles depending on operating conditions.
The electric bus trial is one of the many measures the Mayor has introduced to make London's bus fleet more environment-friendly, including the operation of zero emission hydrogen buses and delivering Europe's largest hybrid bus fleet.
Around eight hundred hybrid buses operate in London with more being introduced in a rolling programme.
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