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Major charities are taking legal advice over proposed lobbying rules which they say could stop them campaigning on public interest issues.

Oxfam and the Royal British Legion are among those complaining that a "lack of clarity" in the lobbying bill could leave them open to prosecution.

It amends rules on what "third parties" can spend ahead of an election, if deemed to be "for election purposes".

Ministers say it only affects groups campaigning for a political party.

But , charities including Shelter, the Salvation Army and the British Heart Foundation complain that the bill is too vague and currently "entirely unworkable".

"We are seeking urgent legal advice in the interests of preventing this bill from having disastrous unintended consequences," they say.

Part of the bill amends existing rules on what groups, which are not political parties, can spend in a general election period on campaigns considered to be "conducted .. for election purposes".

It lowers the maximum that can be spent before groups have to register with the Electoral Commission, from £10,000 to £5,000 in England, and in Scotland, Wales and Northern Ireland from £5,000 to £2,000.

The charities say that the new limits will easily be met by even small organisations - preventing them from taking part in any further activity in that period.

And they fear the rules will impose a "crushing" regulatory burden and risk "seriously hampering their ability to speak up on issues of concern".

They say they are concerned "that the proposed new rules could apply to a range of normal and legitimate awareness-raising activities, despite them being intended to be party-politically neutral".

They give examples including one of a health charity campaigning against smoking, at a time smoking legislation became an election issue.

"The complexity of the legislation, the lack of clarity in drafting, the amount of discretion given to the Electoral Commission in determining how the rules apply, and the remarkably burdensome reporting requirements... will collectively have the result of muting charities and groups of all sorts and sizes on the issues that matter most to them and the people they support."

A Cabinet Office spokeswoman said that charities campaigning without any intention to promote a particular party would be exempt.

"The intention of the bill is to bring greater transparency where third parties campaign in a way which supports a particular political party or its candidates, by requiring expenditure on those campaigns to be fully recorded and disclosed, " she said.

"This bill does not include campaigning by third parties - charities or other organisations - that is not intended to promote or could not reasonably be considered to promote the electoral success of any particular party."

Downing Street has said that Prime Minister David Cameron was open to changes to the plans following "discussion and debate" in the Commons - when MPs return next month.

Shadow Commons Leader Angela Eagle accused the government of planning "to gag charities and campaigners in the run-up to the election" and urged a rethink.

"If they don't, it will be clear that this is a sinister attempt to insulate their record and their policies from legitimate, democratic criticism."

The bill has already been criticised by the TUC as "an outrageous attack on freedom of speech".

has also come in for criticism from Political and Constitutional Reform Committee chairman Graham Allen, who has described it as a "dog's breakfast".