A surge in online political advertising spending during last year's general election shows the need for greater transparency, campaigners say.
The Electoral Reform Society (ERS) estimated the three main UK-wide parties spent more than twice as much on online adverts as on 2017's poll.
A lack of regulation was creating a "Wild West" in need of stronger oversight, it added.
The government called its efforts to reform advertising "world-leading".
Last month, minsters published plans for a "digital imprint" on social media ads, promising "the same transparency" for voters as for election leaflets and posters.
The ERS welcomed these, but said they were "unlikely to be sufficient".
In a report, the campaign group said providing more information to voters about political adverts online represented an "urgent challenge for democracy".
It argued claims over their accuracy were becoming "increasingly prominent" online, where it was easier for pop-up campaigners, as well as established political parties, to influence debate.
The ERS added there had been "several high-profile examples of dishonest or misleading claims" across the political spectrum during the 2019 campaign.
It pointed out existing accuracy rules on commercial adverts did not extend to political campaigning, while donations laws provided only a "minimal" snapshot of how much parties spent online.
What did the report find?
- The study, compiled by academics Katharine Dommett and Sam Power, estimated party spending using the transparency archives of social media firms
- It found the Conservatives, Labour and Lib Dems combined spent around £6m campaigning on Facebook and just under £3m on Google
- The analysis suggested the Conservatives spent comparatively more on Google, backing claims it sought to reach voters through YouTube
- The researchers said 64 non-party groups had registered as official political campaigners for the election, with 46 registering after the poll was announced
- They calculated a total of 88 non-party campaign groups placed 13,197 adverts on Facebook, at a combined cost of £2.7m
The report made recommendations requiring political campaigners to provide more detailed spending invoices more quickly after elections to the Electoral Commission, the UK's elections watchdog.
It also urged parties to work with regulators and the advertising industry to develop a code of practice for political adverts.
The Electoral Commission says it does not have the power or resources to monitor the truthfulness of political advertising.
But it has previously echoed calls for greater transparency, adding in its review of the 2019 election that rules needed to be updated.
Constitution Minister Chloe Smith said: "People want to engage with politics online. That's where campaigners connect with voters, so naturally political parties across the board are increasing their digital campaigning activity.
"This government is already making political campaigning more transparent for voters, with new, world-leading measures that will require campaign content promoted online to explicitly show who is behind it."