Canada's federal and provincial governments reached a limited agreement on changes to the country's constitution yesterday, but the agreement is so fragile that it could easily fall apart in the next three months.

The "package deal," as it was called by Prime Minister Pierre Trudeau, would limit the authority of the central government in selected areas in exchange for provincial consent to changes in the Supreme Court and the protection of human and linguistic rights.

The package deal attempted to meet at least some fundamental demands for reform from all regions of Canada. It will be studied by a committee of officials in the next three months.But deep differences remain between Ottawa and the 10 provinces and among the provinces themselves. These could prevent permanent agreement on changes when Trudeau and the provincial premiers meet again in February.

That conference will be the last before the federal election which Trudeau must call before July. He is in political trouble across English Canada, with a Gallop Poll released yesterday showing his Liberal Party trailing the opposition Conservative Party for the first time in nearly three years.

Thus, Trudeau badly needs an agreement on constitutional reform to convince the electorate that he is the only political leader capable of holding Canada together against the possible secession of French-speaking Quebec Province.

To get an agreement, Trudeau made broad concessions to the powerful provincial leaders. He produced a list of four areas where the federal government was prepared to see its powers restricted. He further agreed to reaffirm provincial control of natural resources, as demanded by the oil-rich western provinces.

But Trudeau's proposal to entrench a charter of human rights in the constitution, including linguistic rights for English and French, was opposed by five premiers who said the result would be that the courts would start settling social and political questions better handled by elected bodies.

These premiers warned that Canada should not follow the U.S. experience and place rights in a written constitution that the court would be free to interpret.

Canada, already the world's most decentralized federation, has been trying for 51 years to bring wholesale changes to its constitution, a collection of statutes of the Canadian and British parliaments.

The most important constitutional document - the British-North American Act of 1867, the year of Canada'a feredation - remains a statute of Great Britain.

Every time Canada wishes to amend this act, it must petition the British government, a process Trudeau describes as a humiliating legacy of conlonislism.

But Canada's federal and provincial governments have never been able to agree on an amending formula that would put Act of 1867 under Canadian law.

Trudeau pressed the premiers for a commitment to find an amending formula. But Quebec Premier Rene Lelinking federal concessions to provincial concurrence on an acceptable formula. But Quebec Premier Rene Levvesque, who wants to lead his province out of Canada, said yesterday he would agree to an amending formula only if Quebec received far more powers than Trudeau was willing to offer.

The impasse between Trudeau and Levesque could scuttle progress but the political imperative for Trudeau and the nine English-speaking premiers is to achieve a concrete agreement, however limited, to show Quebec before it votes in a referendum on secession that the Canadian federal system is flexible.