Tony Blair has been challenged to say whether a strike by tube workers would amount to secondary action in light of on-going action in the fire service.

Ahead of a ballot on Thursday over the safety threat to tube workers during fire strikes, Iain Duncan Smith said the prime was giving a "green light" to militant unions.

He said the government was not sending out a clear message about whether a strike by tube workers would fall foul of industrial relations legislation.

"We have had thousands of teachers and council workers on strike this week, now tube workers and fire station workers are threatening secondary action and the prime minister cannot say whether it's secondary action or not," said Duncan Smith.

"After today's answers from the prime minister doesn't he think that will send the green light to all those who think they have a right to strike?"

The Tory leader said London Underground should use the law to prevent secondary action.

"In short, have they been told to get an injunction?" Duncan Smith asked Blair during a rowdy session of prime minister's questions.

Blair said that "any strike would be totally unjustified" and expressed confidence in London Underground's ability to respond to any illegal action by its workers.

"There must be no return to secondary action or secondary picketing if it is unlawful," the prime minister told MPs.

He said that any decision on the legality of industrial action would be for the courts to make.

Liberal Democrat leader Charles Kennedy tackled the prime minister over the recession that is gripping manufacturing.

He argued that the chancellor had to do more to help stimulate a sector that was now experiencing severe problems.

"Isn't a government that said it would deliver economic stability embarrassed by the fact that Britain is suffering the longest manufacturing recession since the Second World War and that business investment in our country is lower now than during the Tory recessions of the 80s and 90s?" he asked.

Kennedy also pointed to share of EU inward investment that had fallen from 28 per cent four years ago to 16 per cent now.

"How low must it go before the prime minister takes action?"

Blair hit back saying the biggest help was the current low levels of unemployment and inflation and argued the Liberal Democrat plans would wreck the economy.