The breakthrough came in private talks between the Home Secretary and Martin Bangemann, the European Commissioner for the Internal Market, when they discussed ways of resolving the long-running battle over the border issue during the present six-month British presidency of the EC.
Britain's objection to the abolition of its border controls threatened to dominate the meeting of the Trevi group of immigration and home affairs ministers that Mr Clarke is due to chair under the British presidency on 30 November and 1 December in London.
The Home Secretary has told colleagues that Mr Bangemann gave him a private assurance that Britain will not be taken to the European Court of Justice for maintaining its border controls when they are abolished in the rest of Europe next January.
The value of the informal deal is certain to be questioned by anti-federalist Tory MPs, who are opposed to the Maastricht treaty. They regard the threat to Britain's border controls as evidence of the excessive power of the Commission and were outraged when Douglas Hurd, the Foreign Secretary, was unable to give them similar assurances at a backbench meeting in Westminster.
However, while it is unlikely to silence the Tory doubters, ministers believe the deal will help to put the Maastricht treaty back on track for ratification during the British presidency, if the French vote 'yes' to Maastricht in their referendum in September.
Ministers regard the deal as further evidence of the leverage they are achieving during the British presidency. John MacGregor, the Secretary of State for Transport, also secured a deal, during an informal gathering of ministers, for the Commission to drop its threat to intervene in the Twyford Down M3 controversy, although it would continue to challenge the legality of a road through Oxleas Wood in south-east London.
Mr Bangemann's reassurance is seen by ministers as a further signal that the Commission is heeding criticism, following the Danish rejection of Maastricht, that it has been interfering too much in the affairs of member states.
The deal could also have long-term political importance for Mr Bangemann, a vice-president of the Commission in Brussels. With Sir Leon Brittan, he is one of Britain's preferred candidates to replace Jacques Delors as President of the Commission in two years' time.
The border issue has been the subject of a running battle with the Commission for a number of years. The internal EC border checks on passports in the rest of Europe are due to be dropped as part of the Single European Act, which was signed by Baroness Thatcher as Prime Minister. The Act removes customs barriers in January next year to establish the Single European Market, which Britain has taken the lead in supporting. However, the Commission had insisted that the Act ensured the free passage of people as well as goods.
European partners have argued that strict external controls will make the internal borders redundant. But British ministers have privately cast doubt on the effectiveness of immigration controls in southern states such as Greece. They have also detected a shift of opinion among French and German ministers, following large economic migration which has fed European fascist parties.
British ministers have argued that Britain should be treated differently, and should be allowed to maintain passport checks on EC nationals, because of its island status and the increased threat posed by terrorists and drug traffickers. The Commission issued a legal opinion in May that article 8a of the Act required an end to all controls and was confident that if it took Britain to the European Court it would win.
It could remain open to an EC citizen, challenged at a British border control to present a passport, to take Britain to the European Court. But Mr Bangemann's assurances to Mr Clarke suggest that such private action would not have the Commission's support.
It is likely to mean that when EC border controls are relaxed in the rest of Europe, EC visitors to Britain will still be expected to carry passports and will be open to immigration controls. However, to avoid delays, the passports may be subject only to occasional spot checks, like those at Swiss-French border-crossing points.Reuse content