Conservative MPs could rebel over the Government's trade union reforms unless measures which could damage civil liberties are dropped, one of the party's senior MPs has suggested.
David Davis said he was "not alone" in opposing plans to force picket organisers to give their names to the police and to restrict action by unions on social media.
Mr Davis, whose grandfather was blacklisted for 17 years, said he would vote for the Bill at second reading today but suggested he would oppose it with others if the measures are not removed.
The Haltemprice and Howden MP said the move on giving names to the police violated the principle of freedom of association and could lead to a "slippery slope".
He told the Commons: "I particularly am offended by the idea that a picket organiser needs to give his name to the police.
"This to me is a serious restriction of freedom of association.
"It's not on the same basis as let's say getting the organiser of a big demonstration to give his name to the workforce, there's all the difference in the world between half a million people clogging up London and half a dozen pickets standing around a brazier shivering whilst they are trying to maintain a strike.
"It seems to me this is an incredibly important issue and is one that we don't want to get on a slippery slope with and I say to the minister that I will be seeking, and I'm not alone, to alter that during the progress of this Bill."
Mr Davis said plans to regulate what unions post on social media would lead to legal challenges and could not be policed.
He went on: "There are arguments that it's to stop bullying.
"Well fine, pass a law to stop bullying but make it affect everybody, don't just make it affect trade unions. Pass a law to stop intimidation, make it affect everybody.
"We do have quite a lot of law to prevent intimidation.
"Those are two critical elements to me - weaknesses in this Bill.
"I say to you I will seek to prevent both of them making it through to third reading, if they are included in third reading, I'm afraid I'll vote against.
"I'll vote for today but if this Bill has still got those measures in when we get to third reading I'll be voting against.
"I say again to you - I doubt I'll be alone."
Conservative William Wragg, a member of the Association of Teachers and Lecturers, said the Bill contains a number of "sensible" measures but he also voiced concerns.
The Hazel Grove MP said: "I am concerned facility time conflates both trade union duties and trade union activities.
"We need to be clear - this time should not be used for political activity, such as the organisation of strikes and political lobbying, however, it should be for the genuine need of union representatives to be involved in important HR matters between individual union members and employers.
"I'd suggest to the minister that in seeking to reduce such facility time through publication, which doesn't take into account the exact nature of it, there's the risk that genuine HR matters may be unfairly neglected."
He also said he regarded health and safety as a duty of a union rather than an activity.
Mr Wragg went on: "My second concern is with the possibility of the use of agency workers to cover striking workers."
He urged the Government "not to go down that path".
Mr Wragg said he is "minded to support" the Bill's second reading, noting he hopes the minister will listen and consider the arguments put to him.
Conservative Jeremy Lefroy (Stafford) said he could not see the problem with electronic voting, as he raised concerns about the Bill.
He said: "Electronic voting will come in and if we're to ask for higher turnouts, electronic voting, I believe, is a must."
Mr Lefroy said Mr Davis expressed the views on picket lines in a more eloquent way than he could, as he also suggested there is a need to reform political funding across the board.
He said: "It's very difficult to do that without a comprehensive solution.
"I know in the last Parliament this was tried and it did not work but I would urge the Government and the Opposition to sit down and try and sort this out once and for all."
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