Unions with more than 10,000 members will have to submit an annual Membership Audit Certificate to the Certification Officer.
This is on top of the annual return which they already have to make.
The Certification Officer will have the power to require production of “relevant” documents, including membership records and private correspondence.
David Cameron has defended the lobbying bill as it will make it harder for unions to spend money.
He complained that unions “go on spending millions after millions trying to alter an election campaign”.
Cameron said that if the bill became law it would “control” union spending.
TUC general secretary Frances O’Grady opposes the bill.
She said it is “One of the biggest threats to freedom of speech we have seen for years”.
“This is supposed to be a free country where ordinary people are supposed to have a say, not one where we’re gagged because professional politicians don’t like what we’re saying.”
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