Martin McGuinness has rejected criticism of his party's stance on expenses
The Conservatives say that if elected, they would insist no parliamentary allowances be paid to Sinn Fein unless the party took their Westminster seats.
Five Sinn Fein MPs have claimed expenses on two London flats.
It emerged last week that the MPs, who do not take their seats at Westminster, have claimed more than £400,000 in allowances.
The party said it had not broken any rules and its MPs use the flats.
Shadow Northern Ireland Secretary, Owen Paterson, has said he has had private discussions with Sinn Fein over the issue and will continue to meet with them.
He claimed it was "completely unacceptable" for Sinn Fein representatives, "who won't even sit in Parliament, to claim hundreds of thousands at the taxpayers' expense".
A spokesman for Sinn Fein said they would resist any attempt by the Conservatives to "undermine the democratic process in Ireland".
Sinn Fein MPs do not take a salary but have been entitled to allowances since a Commons vote in 2001.
The move was not part of the Good Friday agreement, but the then foreign secretary Robin Cook said: "We are more likely to secure further decommissioning if we demonstrate that we are willing to maintain the momentum on our side."
In the last financial year, Sinn Fein's five MPs Gerry Adams, Martin McGuinness, Pat Doherty, Michelle Gildernew, and Conor Murphy each claimed £21,000 in Additional Cost Allowances - just short of the maximum.
Since the figures were first published in 2001/02, the Sinn Fein MPs, who do not take their seats at Westminster as that would involve swearing allegiance to the Queen, have received a total of £437,405 in the taxpayer-funded allowance.
The payments were strongly condemned by senior Conservatives.
A Sinn Fein spokesman said: "Sinn Fein MPs do not receive a salary from Westminster, nor do they employ any family members. Sinn Fein makes no apology for refusing to sit in the British House of Commons.
"We also make no apology for ensuring that people who vote for Sinn Fein get the same democratic entitlements as everyone else."
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