The Metropolitan police commissioner, Sir Paul Stephenson, denied speculation that senior members of his force are reluctant to become embroiled in another political inquiry.
He told London Evening Standard that some MPs may be best dealt with by the authorities in the House of Commons.
But Stephenson added: "We will not back down where there is an obvious and clear need to investigate and people can be confident about that."
His comments came as senior officers prepared to meet colleagues at the Crown Prosecution Service to decide what action to take.
Stephenson held talks with the director of public prosecutions, Keir Starmer, last week to assess whether criminal inquiries may be necessary.
The Met chief added: "I do not want to get drawn into party political battles. But if there are matters that are brought to my attention that warrant investigation then rest assured we will investigate.
"We will not back away from our responsibilities."
He said that any police investigation must balance the cost to the taxpayer and competing demands of other high-profile crime threats.
Previous political probes including "cash-for-honours", "donorgate" and "lords-for-hire" have not led to criminal prosecutions.
Stephenson also called for a shakeup of how officers are called upon to probe political matters, suggesting an "independent gateway process" to avoid the police being used in any "party political game".
It is likely that any initial police inquiry will centre on two Labour MPs who claimed for mortgages they had already repaid.
Hundreds of members of the public are understood to have made complaints to Scotland Yard as the expenses scandal has unfolded in the media.
Campaign groups have threatened to launch private prosecutions if the Metropolitan police do not investigate.
Among those in the spotlight are Labour ministers including the chancellor, Alistair Darling, the transport secretary, Geoff Hoon, and the employment minister Tony McNulty.
A former environment minister, Elliot Morley, and the backbencher David Chaytor have been suspended by the parliamentary Labour party.
Chaytor was axed after he admitted an "unforgivable error" in receiving ?13,000 to cover interest payments for a loan already paid off.
Scunthorpe MP Morley has also been suspended for a similar ?16,000 claim and has already said he may resign.
Any Met inquiry would be undertaken by detectives from the force's specialist crime directorate, led by the acting assistant commissioner Janet Williams.
Lord Falconer, the former lord chancellor, has warned a police inquiry could last more than two years, potentially overshadowing the next general election.
In a separate development a leading union called on Labour's executive to outline action to deselect MPs found guilty of unjustified expenses claims.
The GMB said its members wanted the NEC to outline the process that will be used to deselect MPs as candidates for the Labour party at the next general election
Speaking ahead of a meeting of Labour's executive tomorrow, the GMB general secretary, Paul Kenny, said: "GMB members, like the rest of the nation, have felt disgust and outrage at the revelations surrounding the claiming and pocketing of expenses by MPs.
"A significant minority of elected MPs have not exercised proper judgment in what they have claimed as expenses while doing their jobs. There can be no excuse for this and there can be no hiding behind the rules ... Those who have failed to exercise this moral judgment have no place in parliament representing the Labour party.
"MPs guilty of this have brought parliament and the Labour party into disrepute. GMB members want the NEC to outline the process that will be used to de-select these MPs as candidates for the Labour party for the next general election."