She said Glen Sannox, for the main Arran route, will not now be finished until between March and May 2023 – five years late.
Its unnamed sister vessel, “hull 802”, earmarked for the Skye-North Uist-Harris triangle, will not be ready until between October and December 2023.
Ms Forbes insisted the vessels “must and will be completed”, but brushed aside calls from Scottish Labour and the Scottish Liberal Democrats to resign if the new dates were not met.
She said the Port Glasgow shipyard building the vessels had said the latest delays were due to problems with previously-installed cables, “the risk of further unknown legacy issues” and recruiting suitable staff.
She said the projected cost had increased by £8.7 million to £122.5m-£126.5m, which, with the original £97m contract would make a total of up to £223.5m.
In a letter to MSPs, Ferguson Marine chief executive David Tydeman said Glen Sannox had suffered from five years of "stop/start” construction, with components such as masts, funnels, pipework and cabling “installed in a far from ideal sequence as design conflicts and challenges were dealt with.
"Full confidence that there are no more ‘legacy issues’ will only be secured as the commissioning programme progresses over the next three to six months.”
The announcement on Wednesday came hours after a damning report by public spending watchdog Audit Scotland said the ferries would cost around £240m – two and half times the original price.
It called into question ministers’ decision to award the contract to Ferguson Marine despite the “severe misgivings” of its experts because of the yard’s inability to provide mandatory financial guarantees.
The report also said “significant operational failures” remained at the yard and further remedial work on the vessels was still required.
Ms Forbes said the shipyard, which was taken over by the Scottish Government in 2019, had “made clear these delays and additional costs are due to outstanding legacy cabling issues”, which were the responsibility of its previous owners.
She said: "I recognise the critical nature of completing the ferries for the sake of island communities and I understand the urgency and necessity of delivering these vessels.”
However, Scottish Labour transport spokesperson Neil Bibby said: “The ship has sailed on SNP excuses” and called on Ms Forbes to step down if the latest completion dates were not achieved.
The demand was echoed by Scottish Liberal Democrats economy spokesperson Willie Rennie.
He said: “The minister tries desperately to distance herself from this ferry fiasco but these ferries were contracted by government-owned CMal [ferrying-buying arm Caledonian Maritime Assets Limited], paid for by the government, built originally by a company celebrated by the government and when it collapsed it was owned by the government.
"Still no minister has ever faced the music.”
Scottish Conservatives transport spokesperson Graham Simpson called for a public inquiry into the SNP’s “epic incompetence”.
He said: “Ministers and others senior figures have moved on but nobody’s head has rolled.
"There is no accountability - not just at Ferguson Marine, but in the entire ferry system and, especially in Government.”