A quarter of projects supported by the Heritage Lottery Fund were finished late and one in six over-ran on costs, a spending watchdog has said.
Poor planning contributed to delays in projects, the report said
The National Audit Office said about 17% of completed schemes went over budget by an average of £293,000.
It said time over-runs were mainly due to "unanticipated" events and poor planning, while inflation was a major cause of projects running over budget.
The fund said it had taken "significant steps" to improve the situation.
The Heritage Lottery Fund distributes around one sixth of the money raised by the National Lottery for good causes.
It gives grants to projects involving the local, regional and national heritage of the UK.
The audit office's findings were based on a review of 30 funded projects, and a survey of 2,372 grant applicants.
It said some 6% of projects got extra grants at an average of £176,000 to meet costs.
More than half of the projects would not have gone ahead without financial support from the fund, applicants said.
Improved management skills on some projects was needed, the NAO said.
NAO chief Sir John Bourn said: "The Heritage Lottery Fund has done valuable work in preserving the UK's heritage and promoting access to it.
"However, it needs to do more to simplify its procedures, to provide better support to applicants and grant recipients, and to reduce time and cost over-runs."
Commons Public Accounts Committee chairman Edward Leigh said the fund had done a lot of good work but must not be complacent.
He said: "From 2009, the fund will have to deal with its money being cut by a third each year largely as a consequence of Lottery funding of the Olympic Games.
"We must be alert to the danger that by looking to the future we may forget our past. The fund will have to work hard to manage this reduced budget."
The fund said in a statement: "The HLF has already taken significant steps to ensure areas highlighted for improvement are being tackled head-on.
"Most importantly, this is involving making sure the application process is simplified and that overall assistance for applicants is as helpful as possible."
The fund has distributed £3.8bn to about 24,000 projects since it started in 1994.