A British demand for a further "opt out" - in addition to those related to the single currency and the social chapter - would re-ignite the Government's conflict with Europe in the run-up to the election. It would also win widespread backing from Tory Eurosceptics, while seriously souring negotiations at the inter-governmental conference on European Union reform.
According to senior Whitehall sources, Mr Major's support for a new opt-out would help defuse the political storm that could erupt if the European Court of Justice rejects a British challenge to the working hours directive in September.
The Government is arguing that the EU has brought the law on the working week in by the "back-door", using health and safety legislation, instead of the social chapter provisions under which Britain would have had a let-out. However, the claim carries little credibility as the directive was drawn up as a health and safety measure well before the advent of Britain's social "opt-out".
If the Government is defeated in the court, Mr Major will come under tremendous pressure from his backbenchers to side-step the working week ruling, which Euro-sceptics view as blatant interference from Brussels. The Prime Minister faces the embarrassing prospect of having to implement thedirective by making changes toBritish domestic law in the run up to the general election.
The Government is therefore urgently considering how it can counter the new European ruling with political weapons in the context of the IGC negotiations. One possibility, say senior officials, would be to persuade other EU member states that the health and safety treaty Article 118a - under which the working hours directive was implemented - should be re-written so that it can not be used to affect working practices.
However, Mr Major has been advised that the only way of ensuring that no such further measures are brought in under Article 118a, would be to opt out of this piece of the EU treaty altogether. Such a move would be risky and would create enormous controversy for the Prime Minister at home and on the Continent.
Britain's agreement to Article 118a stems from Baroness Thatcher's signing of the Single European Act in 1986. Britain has always argued for strong health and safety legislation. Most European legal experts, including senior government advisors, believe there is a good legal case for arguing that limiting working hours is a matter of health and safety.
To ask for a retrospective opt-out 10 years on would be unprecedented. Up to now opt- outs have only been negotiated for prospective European laws. Apart from this, British calls for an opt-out will be greeted by howls of protest from the other member states. Negotiation on the working hours directive began in 1990, by which time member states had already agreed to British demands for watering down the measures.
Mr Major could argue that a further British "opt-out" is in line with Europe's new drive for "flexibility" - meaning allowing some countries to move towards integration faster than others. However, most other states are strongly against an approach which allows one member state to choose which EU politicies it likes and which it wants to reject. Senior Commission officials, meanwhile, speculate that if Britain continues to reject key EU provisions,member states may ask whether the country should remain part of the single market.