Ramblers fear they could be imprisoned for up to a year for walking in parts of the countryside under Government plans to crack down on trespassing.
The Ramblers' Association said the Serious Organised Crime and Police Bill, to be discussed in the Commons today, plans to criminalise trespassing on land owned by the Royal Family.
The move follows recent security breaches at Buckingham Palace and Windsor Castle but walkers fear the legislation will apply to all land owned by the Crown, including parts of Dartmoor in Devon and the Royal Parks.
The legislation would automatically designate Crown and royal residences and the land close by as no-go areas, effectively introducing a law of trespass claim critics.
Royal residences would become off-limits to everyone except those with special permission.
Janet Davis, head of the association's footpath policy team, said: "This is a clear case of the Government using a sledgehammer to crack a nut.
"The Government claims it is intending to 'attack the particular mischief of people getting into Buckingham Palace and Windsor Castle' but refuses to limit the Bill to these and other Royal residences.
"It is instructive that the Scottish Parliament has removed these clauses, while at Westminster the Government says one thing and does another.
"The Bill raises the spectre of walkers being made criminals for no other reason than the land involved is Crown land. Currently trespass is a civil offence; this Government seems determined to make it a criminal offence on designated Crown land of unlimited size and for an unlimited length of time.
"Even more worryingly, it would appear from the wording of the Bill that there will be a presumption of guilt for anyone found walking on the land."
The organisation claims the legislation would affect huge areas of land from Northumberland to Dartmoor in Devon and could even extend to the Royal Parks and parts of the Thames Path in London.