Lord Williams of Elvel, the Opposition spokesman, said in the Lords that he and Earl Howe, a junior agriculture minister, were recipients of such grants but did not say whether he believed overpayments had been made in either case.
He told peers: "Contracts awarded to landowners in environmentally sensitive areas are based on Ordnance Survey maps, and the OS map, for instance, on Radnor is way out of date. Nevertheless, public money is being paid out to landowners ... on the basis of out-of-date information.
"I am talking about public money which is paid out to landowners, such as myself and, as I understand it, Earl Howe, on the basis of out-of-date OS maps in rural areas."
He urged the Government to ensure maps were rectified.
The Earl of Lindsay, for ministers, replied that maps of field boundaries which had not been revised since the mid 1970s would be updated in the next five years.
It was for grant applicants to ensure they provided accurate information about acreages. They could, if necessary, commission a customised map from the Ordnance Survey, but added: "There is no excuse for inaccurate OS information."
Lord Williams protested: "If the Government gets it wrong, I am liable. I don't believe that's right. If the Government makes a contract, should not the Government meet that contract?"
Lord Pearson of Rannoch, a Tory Euro-sceptic, said the OS map of Rannoch Moor in the Scottish Highlands did not make sufficiently clear whether heights were in feet or metres. "Some maps suggest that a hill is only 300m high when in fact it is over 1,000ft," he protested. "This is quite dangerous for hikers in winter."
The Ministry of Agriculturelater said Lord Howe's farming interests were looked after by managers in the same way as those of William Waldegrave, the Minister for Agriculture.
The Earl of Clanwilliam said at question time that now the RAF's atomic bomb area at Chilmark in Wiltshire had been closed and dismantled, it should be shown on the area's OS map.
A spokesman for the Ministry of Defence later said Chilmark had been an ammunition depot, but would not confirm whether nuclear weapons had been stored there.