The revelation, which contrasts with Mr Blair's statement that he did not give money to beggars, comes one day ahead of a charity launch that will highlight homelessness. Both party leaders will be present.
In an interview in Monday's edition of the Big Issue, the magazine sold by homeless people, Mr Ashdown recalls taking a teenage man and a woman in her twenties into his home in Yeovil, Somerset.
He saw them while walking home past a derelict house where a fire was burning. Because of the extreme cold he invited both of them to his home, in which they stayed for three days.
Mr Ashdown's reputation as a Good Samaritan, however, may be blunted by the fact that the incident took place in 1981, and, when he saw the couple some time later, neither had prospered. The man had, apparently, got into trouble through petty crime, and the woman, who by then was pregnant, may also have had a drugs problem.
As the Liberal Democrat leader said last night: "Seeing at first hand the situation of people in such a desperate situation left a lasting impression.
"What it taught me was that providing someone with a roof over their heads for three nights is not enough. You need a whole structure of support giving people ladders out of homelessness and access to healthcare, counselling, training and support."
This week Mr Ashdown will propose a national empty-homes strategy giving councils the power to make available to the homeless properties left empty without good reason for 12 months. Landlords would be compensated.
In an earlier interview with the Big Issue, Mr Blair voiced his support for the hard-line "zero tolerance" measures against street crime pioneered in the US, as well as saying he did not give beggars money.
John Major, likely to be the Big Issue's next interview target, has so far restricted himself to saying that he has given "millions of pounds" of taxpayers' money to set up shelters for the homeless.Reuse content