Conservatives slammed for banking almost £4m from anonymous donors since 2016
The Tories accepted almost £4m in cash from anonymous donor groups while Theresa May was leader, Total Politics can reveal.
The party was accused of "obscuring dark money" donations after accepting over £3.9m in cash from opaque groups, including dinner clubs and business forums.
Under current electoral finance laws, parties are permitted to accept donations from groups set up as "unincorporated associations" without having to publish the details of individual donors.
The method, used by most major UK parties, allows constituency groups and local councillors to donate cash without having to register as a limited company.
But an analysis of data from the Electoral Commission shows significant sums donated to the Conservative Party have come from obscure organisations such as "The Sixty Six Club", and "Strangers Gallery".
Another group, Businessfore, has donated £85,000 to local Conservative groups in Watford and Burton since 2016, while also donating a further £10,000 to Tory MP Andrew Griffiths.
But the group, registered to a residential address in Chelsea, has no website and little public information is available online about its activities.
Neither Mr Griffiths or Watford MP Richard Harrington responded to questions about the donations.
SNP MP Martin Docherty-Hughes said the party had "serious questions" to answer over the donations.
"At a time when public trust in the integrity of the UK's democratic processes is being eroded, it's not credible for the Tories to continue obscuring their donations from public scrutiny," he told Total Politics.
"The public must have faith that our democracy is not being bought, but unincorporated associations are being used to allow huge sums of dark money to be pumped into our political process.
He added: "We need transparency over political donations and it's about time the Tories were honest with the electorate about who funds their party."
Often described as "business clubs" and registered to the local Conservative Association offices, the loophole allows supporters to provide funds to the party without having to register as individual donors.
Steve Goodrich, a research manager at charity Transparency International UK, said: "A cornerstone of our campaigning finance laws is shining a light on where big money comes from and where it goes.
"When the public are left in the dark about this, it suggests the rules either aren't being followed or aren't achieving what they set out to do."
He added: "There are serious questions as to whether there is sufficient transparency over how unincorporated associations fund their donations to political parties.
"Although a number of these groups are made up of councillors, many of the biggest are associations whose funding and operations are unclear."
A Conservative Party spokesperson said: "All major political parties receive donations from unincorporated associations, which are properly and transparently declared to the Electoral Commission, published by them, and comply fully with the law."