ONGC is near 90 per cent acceptance of its 1.3 billion pounds ($1.88 billion) bid for Imperial Energy, the threshold at which it will be forced to complete the deal, a source familiar with the matter said on Tuesday.Imperial investors, bankers and analysts believe ONGC will back out of the takeover if shareholders representing 90 per cent of Imperial's share capital do no accept the bid by 1300 GMT on Tuesday.The threshold was not met by the close of business on Monday but the source said close to 90 per cent had accepted and that Imperial expects the threshold to be met by the bid deadline."They are very close to the finish line," the source said.ONGC agreed the deal in August when crude was trading at around $130/barrel, compared to under $40/bbl now.Imperial shares traded up 2 per cent at 1052 pence at 0917 GMT.RBC Capital Markets predicted last week that if the deal was to collapse, the shares would fall to 270-500p/share, based on the recent share performance of other exploration companies operating in the former Soviet Union.ONGC Videsh, the ONGC unit which is making the bid, declined comment, as did Imperial.In a takeover, investors often wait to tender their shares to a bidder at the last moment, in the hope a higher bidder will emerge but Imperial and its advisors have been working hard to convince shareholders takeovers to accept the offer.