LONDON (AP) _ Today is A-Day in the City of London - the appointed day that the core of a sweeping new system for regulating the financial services industry takes effect, touching almost everyone involved in the $43 billion business.

The new regulations, imposed under the Financial Services Act of 1986, are aimed at better protecting investors.

But industry critics call the new system a confusing hodge-podge of vague regulations that could cost hundreds of millions of pounds to implement, are hard to comply with and could drive business out of Britain and into other financial centers.

''We think that is a very real fear,'' said Robin Hutton, director general of the British Merchant Banking and Securities Houses Association.

Consumer advocates claim the regulations don't provide enough protection for investors and that their costs will be passed on to consumers.

While the act's major protections take effect today, the long-awaited rules are being introduced piecemeal and some have been postponed.

The system replaces a former combination of government regulation, self- regulation, and in some areas - such as the giving of investment advice - no regulation at all.

''It plugs quite a few gaps,'' said Barbara Conway, a spokeswoman for the new regulatory system.

Prime Minister Margaret Thatcher's Conservative government, criticized for its alleged laissez-faire attitude toward financial fraud, has been trying to determine how best to protect investors while overseeig the financial services industry, which it deregulated under the so-called ''Big Bang'' in October 1986.

The government has transferred responsibility for industry regulation to the 17-member Securities and Investments Board, a new non-government watchdog body with statutory powers.

The board in turn has approved five self-regulatory organizations covering different financial services sectors, which will authorize firms and individuals to operate. These so-called ''SROs'' will police the approved firms, which will be required to comply with tough capital-adequacy regulations.

The rules require investment advisers to be more knowledgeable about their clients' finances, investment goals and willingness to take risks.

Failure of firms and individuals to obtain SRO authorization constitutes a criminal offense carrying a maximum sentence of two years in prison and unlimited fine.

Under the Big Bang deregulation - most of which took place Oct. 27, 1986 - brokers were allowed to make markets in stocks, an activity previously restricted to firms called jobbers.

Deregulation also abolished minimum commissions and allowed banks to enter the securities business, among other things.

But less than two months after Big Bang, stock market scandals involving Guinness PLC and others broke, prompting criticism that the City - London's financial district - was not being policed properly.

In the Guinness scandal, seven men including Guinness' former chairman and chief executive officer were charged after Guinness disclosed that its former management manipulated the company's stock price to make a 1986 cash-and-stock offer for Distillers Co. more attractive than a rival bid.

Hutton, of the industry association, said ''It is wrong to think that the new measures will prevent fraud and do what the politicians want, which is to catch crooks more quickly.''

There are other gaps. For example, the act doesn't cover Lloyds of London, the self-regulating insurance market.

Furthermore, provisions for compensating clients who have lost money through the collapse of firms won't go into effect for months.

''What we're saying to investors is 'Don't be lulled into a sense of security,''' said Helena Wiesner, a research manager at Britain's Consumers Association.

More than 5,000 of the industry's players, or more than 10 percent, have not yet obtained SRO approval to operate. The approval process was so cumbersome that SROs have granted interim authorization to firms to meet today's deadline.

Hutton said: ''If and when we get the financial services regime up and running...we should have a very advanced system of regulation in this country that will provide an increased measure of protection for investors.

How long could that take?

''Something like two years,'' he said.