Police are “carefully” assessing more than 900 documents to determine whether or not crimes were committed by Leave campaigns during the EU referendum.
Met Police chief Cressida Dick said they had received a “huge amount of material”, which was being considered.
It would be wrong to comment on whether a police inquiry would be launched at this early “scoping” stage, she said.
The Electoral Commission has referred both Vote Leave and Leave.EU to police over potential electoral law breaches.
Leave.EU, which was backed by then UKIP leader Nigel Farage, lost out to Vote Leave, which included Michael Gove and Boris Johnson among supporters, in the battle to become the official Leave campaign in the 2016 EU referendum.
Both groups have denied wrongdoing and accused the commission of a politically-motivated attack.
On Thursday, the National Crime Agency confirmed it was investigating Leave.EU and its co-founder Arron Banks and chief executive Liz Bilney over separate allegations that it had breached electoral law by accepting loans from “impermissible sources”.
Mr Banks said he welcomed the investigation and was confident it would “put an end to the ludicrous allegations”.
In May, the Commission fined Leave.EU £70,000, saying it had exceeded its spending limit by at least 10% by failing to include at least £77,380 in its spending return. It referred Leave.EU chief executive Liz Bilney to the police.
In July, the elections watchdog fined Vote Leave £61,000 and referred the campaign to the Met, saying it had exceeded its £7m spending limit by funnelling £675,315 through pro-Brexit youth group BeLeave.
The Met has denied reported claims that “political sensitivities” had delayed the Met’s probe.
The force says it was not until 7 September that it received “over 900 documents” relating to both referrals from the Electoral Commission.
On Friday, the Metropolitan Police Commissioner told BBC Radio 4’s Today programme: “We have been passed by the Electoral Commission, on two separate occasions, a huge amount of material.”
“They pass us material when they think it may be a matter for the police, there may have been some crime committed and we are assessing that material and we are doing that carefully.
“It’s just at a scoping stage so it would be wrong for me to say, before we know what we have got properly in the material and what it amounts to, whether there is any criminal investigation there or not. At the moment we are just assessing that and we will do that properly.”
Vote Leave said the Electoral Commission’s report was “wholly inaccurate” and politically motivated, accusing it of basing its conclusions on “unfounded claims and conspiracy theories”.
Leave.EU co-founder Arron Banks has described the Commission’s findings as a “politically motivated attack” and said he would challenge them in court.
Some Remain supporters have suggested Brexit should be put on hold pending the outcome of the National Crime Agency inquiry.
The UK’s referendum in June 2016 was about whether the UK should Remain or Leave the European Union. The result was that the UK voted by 51.9% to 48.1% to leave the EU. This departure is due to happen on 29 March 2019.