Stephen Harper says a Conservative government would double the tax credit for children's fitness activities and introduce a new fitness tax credit targeting adults.

The children's fitness tax credit would be bumped to $1,000 from $500 and a new $500 fitness tax credit for adults would be introduced once the budget is balanced, which is expected to occur around 2015.

"A re-elected Conservative government will enact both of these measures before the end of our next mandate, because our approach is clear — when we can afford it, we will lower your taxes," Harper said during an event at an Ottawa-area gym.

An expert panel would determine which athletic activities qualify for the tax break, according to a Conservative party statement.

"This will lower the cost of those New Year's resolutions," Harper said, as a group of children practiced martial arts behind him. "It will help Canadians make physical activity a larger part of their lives."

Under the current children's fitness tax credit, parents can claim up to $500 in eligible expenses for children under 16, for a maximum $75 rebate. That amount would double to $150 within a Conservative mandate.

The Conservatives said an extended children's program would cost an additional $30 million a year, bringing the total annual cost to $145 million. The adult program would cost $275 million annually, the party estimated.

Harper said the existing tax credit, which was introduced in 2007, benefits roughly 1.4 million Canadian families each year.

After his announcement in Ottawa, Harper is scheduled to travel to London, Ont., where he will appear at a rally.

Harper's news conference came just an hour and a half before Liberal Leader Michael Ignatieff unveiled his party's full platform at a town hall that was streamed on the internet.

At the centre of the Liberal platform is a five-point plan to strengthen families, called the "Liberal Family Pack."

The Liberal leader drew cheers from the crowd when he criticized the Conservative's delayed pledges, saying "We can do it now."