Britain braced for more labor unrest today after a decision to strike by a mine supervisors' union threatened to shut down all of the country's coal mines next week and Prime Minister Margaret Thatcher vowed that the right to close unprofitable mines "cannot and will not be surrendered."

These developments took place against a backdrop of renewed outbreaks of violence on picket lines and clashes between striking miners and police at several mining communities in the English midlands.

News of the new strike, which would begin on Oct. 25, helped send the London stock market tumbling to one of its largest single-day losses. This contributed to the continuing slide of the British pound to record lows against the U.S. dollar and further declines against other European currencies.

Financial analysts in one of London's most respected stockbrokerage firms forecast that the new strike could reduce coal supplies enough to cause power shortages by December. The government disputes such claims and says it will burn more oil and import coal from Poland if necessary. Such prospects, nevertheless, revive memories of power shortages caused by a 1973 miners' strike that helped drive a previous Conservative government from power.

The decision of the coal-pit deputies and supervisors to strike means that all of Britain's 175 coal mining pits could shut down.

About three-fourths of those pits have been shut since the National Union of Mineworkers called a national strike on March 12. But because the national union leader, Arthur Scargill, called that strike without polling the membership, about 50,000 of Britain's 180,000 miners have continued to work.

If the 17,000 pit deputies walk out, however, they could shut all the working mines because the deputies are responsible for security and safety. Legally, the mines cannot operate without them.

The deputies' union is normally the most moderate mining union; it has not voted to strike in 100 years. But union officials said today they were driven to action by the behavior in negotiations of Ian MacGregor, the chairman of the state-run National Coal Board that represents the industry.

The general secretary of the supervisors' union, Peter McNestry, said "only a complete change of attitude from the Coal Board could prevent the strike" from starting next Thursday.

While such a change by the coal board is unlikely, the key question now, union officials acknowledge, is whether the deputies from Nottinghamshire will walk off their jobs and whether the 27,000 miners in Nottinghamshire, who have worked steadily throughout the strike, will keep working without the deputies.

Top aides to Thatcher said privately that nobody really knows if the supervisors' union walkout will be solid at this point. However, union leaders claimed that the Nottinghamshire deputies had voted unanimously to join the strike.

The deputies' walkout was prompted by the coal board several weeks ago when it sought to cut off the pay of any deputy who did not attempt to cross picket lines at struck pits. This outraged the deputies, who voted earlier this month threatening to strike. The vote today sets it in motion.

On the basic issue of the broader miners' strike, namely the authority of the coal board to close unprofitable mines, Thatcher was unwavering in her determination that Britian's coal industry must be made more efficient and competitive and that losses through subsidies of more than $1 billion a year must be shaved.