Failure to secure an adequate compromise when European Union ministers resume the deadlocked talks will give further ammunition to Euro-sceptic Tories as well as angering fishermen in the South-west.
Spain, with the largest fishing fleet in the EU, wants full access to EU waters from 1996, including the 92,000 sq miles from Scotland to Cornwall known as the "Irish Box''.
Negotiations today will focus on three areas within the box which Mr Waldegrave will try to preserve for British and Irish boats. Crucial for the South-west fisherman are the waters between Cornwall and southern Ireland. Spain, however, has threatened toblock entry to the EU of new members Austria, Sweden and Finland, if access is not granted.
David Harris, Conservative MP for St Ives, accused Spain of "blackmail". He and two other Tories representing Cornwall again urged Mr Waldegrave not to sign any compromise damaging to local fishermen.
"If they do go along with such a compromise we would be the first to criticise the Government and we would not support them if there was a subsequent vote in the Commons," he said.
A former MEP who takes a pragmatic approach to the EU, Mr Harris added that the dispute was playing "right into the hands of Euro-sceptics'' and risked becoming much bigger than one simply about fishing.
One of the principal sceptics, Bill Cash, Tory MP for Stafford, predicted the EU council would give in to Spain just as, he said, it had to Italy over milk quota fraud. Britain lost out every time, whether in the Council of Ministers or the courts, he said.
As voting in the Council of Ministers will be by qualified majority, Britain and Ireland require the support of one large country, such as France or Germany, to secure an acceptable deal.
In a further irritant from Brussels yesterday, Britain was ordered by the European Commission to pay back £2.5m for inefficiency and accounting errors involving farm grants and subsidies.