The union has pledged support for its Gibraltarian members who view comments made by Foreign Secretary Jack Straw as an indication that the Government is preparing to allow Spain a greater role in Gibraltar’s affairs.

On behalf of the union, Lord Brett, former IPMS general secretary, will visit Gibraltar to meet with Prospect branch officers and Government representatives. Prospect will also be raising members’ concerns through other political avenues.

In particular, members view with unease Mr Straw’s answer to a Commons question which referred to Spain’s sovereignty claim over Gibraltar’s isthmus without asserting confidence in Britain’s claim.

Mr Straw also said any deal between the British and Spanish governments would be put to a referendum "insofar as it involves a transfer of sovereignty".

Gibraltarians are fearful that this could imply the possibility of mechanisms to give Spain a role in the three-mile long, three-quarter mile wide outcrop without actually transferring legal sovereignty.

Paul Noon, Prospect joint general secretary, said: "Our members on the Rock are just as important as those in other parts of the union. We will do whatever is necessary to defend their interests in this period of uncertainty. Our members are British citizens and their interests must be taken into account in the present talks."

In a letter sent to the Foreign Secretary Noon says: "Continued citizenship in Gibraltar is not incompatible with European Union obligations, nor with continued prosperity for the Gibraltar economy.

"You must know that our members in Gibraltar, as with all the people of Gibraltar, would not accept Spanish sovereignty. They strongly believe that there should be no change to the constitutional position, commitment and priorities without their consent and we strongly support them in this."

Lord Brett said: "I think members in Gibraltar are right to be upset at recent talks between the British and Spanish governments.

"While I have no doubt about the integrity of the ministers concerned, the Foreign Office has a long and ignoble record of selling out British colonies and possessions when they have ceased to be of strategic value.

"Gibraltarians deserve something much better than that and I and a number of other parliamentarians are keeping an eye on discussions and will be anxious that the interests of Gibraltar are not undermined."