Balfour Beatty managing director Paul Lester brushed off doubts over the wisdom of the move, saying BuildOnline had good prospects.
He said Balfour Beatty's contribution was more a symbol of support than an investment, adding that the £10m from ETF and BT guaranteed its future.
He said: "We didn't want to be aligned with someone that wasn't going to be around for long. We looked at a lot of dotcoms but this is the one we went for because we believe it is on top of it and has got £10m in the bank and we like that."