* This year's deficit will be much larger than planned

* Government blames slowdown, lower tax revenues

* Says not changing timetable for return to surplus

* Canadian dollar barely affected by the news (Adds further details)

By Louise Egan

MEECH LAKE, Quebec, May 25 (Reuters) - Canada's budget deficit will be "substantially more" than the C$33.7 billion ($30.1 billion) forecast in January because the economic slowdown is sapping tax revenue, Finance Minister Jim Flaherty warned on Monday.

The acknowledgement, which had little impact the Canadian dollar and other markets, came after many economists and opposition figures complained the government's forecasts were hopelessly optimistic.

Flaherty said higher payments to the unemployed were also to blame for the shortfall and Ottawa will revise its economic growth forecasts.

"The deficit will be substantially more than was anticipated in the January budget and I will report in detail on that in June," Flaherty told reporters.