"The December 1999 all-time has become a major psychological barrier for investors in recent years. In the past two years The FTSE 100 has repeatedly come close to reaching a new all-time high and on 20 separate trading days it has come within 100 points," Adrian Lowcock, head of investing at AXA Wealth, said in note.
News on Friday that euro zone negotiators agreed to extend Greece's bailout package also helps explain the strong gains in the index, analysts said.
"An agreement with Greece to extend its current bailout by four months, a boost to the U.K. economy from lower oil prices and improved economic outlook could mean that 2015 is finally the year the FTSE 100 breaks through the barrier. Once through, the market may well go on to set repeatedly new highs as we have seen happen in both the S&P 500 and the U.K.'s own FTSE 250 in recent years," he added.