The vehicles, which come in various sizes, will distribute post from the central London depot.
They were produced by Oxfordshire-based carmaker Arrival, which recently opened a new factory in Banbury.
Another batch of 100 electric vans for Royal Mail has also been ordered from Peugeot.
Having already been trialled by Royal Mail, these will enter service in December.
Arrival says the nine vehicles it has supplied come in sizes of three and a half, six and seven and a half tonnes.
Although the current prototype versions of the vans are fitted with wing mirrors, a spokeswoman for Arrival told the BBC they will be removed from the final design - as cameras will be used to monitor traffic to the rear instead.
Royal Mail was "delighted" to be collaborating with Arrival, said Paul Gatti, Royal Mail Fleet's managing director.
"We will be putting them through their paces over the next several months to see how they cope with the mail collection demands from our larger sites," he added.
Royal Mail's fleet comprises about 49,000 vehicles in total.
"European postal providers, starting with DHL in Germany, have been taking this approach for a while now," said Ananth Srinivasan, a mobility expert with research consultancy Frost & Sullivan.
He added that five countries - the UK, Germany, France, Italy and Spain - had the potential to convert significant proportions of their fleets to electric vehicles in the coming years.
"Our research indicates that by 2025 these countries may have 100,000 postal service vehicles going electric between them," he told the BBC.

MEP Patrick O'Flynn said the party's support had broadened in the final days of the campaign and suggested their vote share would be more than 10%.
Earlier, deputy chairman Paul Nuttall said UKIP MPs could offer vote-by-vote support to a minority government.
But he told the BBC a referendum on the UK staying in the EU was a "red line".
UKIP is attracting around 14% support, according to the latest BBC poll of polls.
The party had two MPs at the end of the last Parliament - both of whom won by-elections after defecting from the Conservatives.
The party's leader, Euro-MP Nigel Farage, is standing for election to the House of Commons in the Kent constituency of Thanet South. Here is a full list of the candidates contesting the seat.
Speaking about UKIP's prospects, economy spokesman Mr O'Flynn said the party had managed to defy expectations of a squeeze.
He said: "Tomorrow we will significantly and perhaps dramatically outperform the expectations of pollsters and pundits alike.
What are the top issues for each political party at the 2015 general election?
Policy guide: Where the parties stand
"That means doing much better than a political class consensus of a vote share of around 10% and one or two seats in the Commons.
"The so-called squeeze on the UKIP vote predicted by so many of the media class simply has not happened.
"On the contrary we are getting indications of a very substantial energising and broadening of our support in the final days of the campaign."
Speaking on BBC Radio 4's Today programme, Mr Nuttall repeated that his party would not enter a formal deal after the election.
He said: "We've made it perfectly clear there will no coalitions, there will be no pacts, and if we did go down the line of a deal whereby we vote with the government on a vote-by-vote basis, it will be a loose deal."
He said a "red line" would be a "free, full, fair referendum" on EU membership "as soon as possible, preferably before the end of the year."
Mr Nuttall was asked whether a vote for his party could harm the prospects of Tory success and therefore an vote on the EU.
He said: "The problem you've got with Mr Cameron's promise of a referendum is one he's promised a referendum before and gone back on that deal and secondly he's talking about a negotiation through 2016 and into 2017 with an organisation that has made it perfectly clear that renegotiation isn't on the table."
He added: "We're looking to get as many votes as possible tomorrow to ensure we have real change in British politics, whether that's with the European Union or indeed electoral reform."
The best of BBC News' Election 2015 specials
Royal Mail is beginning trials in London of nine fully electric vans with ranges of up to 100 miles.
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The UK Independence Party (UKIP) will "significantly and perhaps dramatically outperform" expectations in Thursday's election, a spokesman has predicted.