FACTBOX - Controversy on detaining terrorism suspects

(Reuters) - The government faces a crucial test in parliament on Wednesday over its plans to extend the length of time that police can hold and question terrorism suspects before having to charge or release them.

Defeat in the vote would seriously undermine Prime Minister Gordon Brown, whose proposals have drawn widespread criticism from lawyers and human rights campaigners.

A poll conducted by YouGov for Wednesday's Daily Telegraph newspaper showed 69 percent public support for Brown's plan to extend the maximum detention period to 42 days "in exceptional circumstances". About a quarter (24 percent) oppose the plan.

Following are some of the issues surrounding the debate.

WHAT IS THE GOVERNMENT PROPOSING?

It wants to extend the maximum period of pre-charge detention, as it is known, from 28 to 42 days. However, it says the 42 days would only be used in rare cases when a "very grave and exceptional terrorist threat" exists. Any use of the new powers would have to be approved by parliament.

WHY DOES IT WANT TO INCREASE THE TIME LIMIT?

Al Qaeda-linked suicide bombers killed 52 people in London in 2005 and the authorities say they have foiled several other major plots. The official threat level remains "severe", defined to mean "an attack is highly likely". Investigations often require analysing huge amounts of seized computer material, which may be encrypted or in other languages, and suspects may have foreign links and use multiple false identities. All this, the government says, means a situation could arise where police have to release a dangerous terrorist suspect because they do not have sufficient time to collect the evidence needed to bring charges. It wants to act now before that happens.

WHAT ARE THE ARGUMENTS AGAINST?