The top four teams from the four highest-ranked European leagues will automatically go into the Champions League group stage from 2018, football's governing body UEFA said Friday.
A new access system for both the Champions League and Europa League will also judge clubs on their own records including historical success in the competitions.
The new format, plus increased financial distribution to clubs in both competitions, comes in reaction to pressure by Europe's top clubs, who had been threatening a breakaway European super league.
At present Spain, Germany, England and Italy are the top-ranked domestic leagues and would be guaranteed four teams in the group stage under the new format.
Spain, Germany and England currently have three teams in the group stage and one in the play-off round. Italy have two in the group stage and one in the play-offs.
The full details of changes for both competitions will be finalised by the end of the year, UEFA said.
The Champions League will continue to have a 32-team group stage leading to a 16-club knockout phase, while the Europa League remains at 48 teams.
The new Champions League format will, however, mean that 16 instead of 11 of the 32 places will definitely be filled by the top four nations and access via qualification and play-off will be more difficult.
It will mean fewer guaranteed places for other associations, with, for example, nations in 11th and 12th-ranked places - currently Czech Republic and Switzerland - without a certain group-stage place.
The European Club Association (ECA) will have a greater voice in a subsidiary company to play a key role in determining the future and the managing of European club competitions.
Half the managing directors in the company, UEFA Club Competitions SA, will be appointed by UEFA and the other half by the ECA.
ECA chairman Karl-Heinz Rummenigge said the UEFA decision "reflects a serious and fair solution for European club football."
He added: "I am particularly pleased with the fact that the European football community remains united moving forward."
UEFA general secretary Theodore Theodoridis said: "The amendments made will continue to ensure qualification based on sporting merit, and the right of all associations and their clubs to compete in Europe's elite club competitions."
On finance, UEFA said sporting performances will be better rewarded but market pool share will decrease.
In response to claims it will lead to clubs from bigger countries getting richer, UEFA said in a Q&A on its website the new revenue distribution model "guarantees an increase in payments to leagues and clubs who are knocked out in the qualifying phase."
All clubs "will receive more money for sporting success and less for just being in a large television market."
UEFA does not say whether the format changes could lead to an association having as many as six teams in the Champions League group stage following qualification.
At present the number of teams per association is capped at five, which could include also the Europa League winner. Any change "will be determined in due course," UEFA said.