Jacob Rees-Mogg has rejected calls from Labour for the Commons to return to a "virtual Parliament", despite the rise in coronavirus cases.
Labour's Valerie Vaz said the country was "entering a really difficult phase" and some MPs were "doing the right thing" staying away from Westminster.
But Mr Mogg said not to treat Parliament as an essential service would be "an offence to democracy".
London is one of several areas set for Covid alert level rises on Saturday.
A "hybrid Parliament" was introduced back in April and some elements of the virtual set up still remain.
MPs can contribute to debates by video link but they are now expected to vote in person - or by proxy - rather than online.
Two unions representing staff in Parliament have now written to the Commons authorities to echo Labour's call.
FDA general secretary Amy Leversidge said: "It is absolutely vital that MPs are able to effectively hold the government to account in the coming months. However, the House authorities also have a duty of care to all those who work on the parliamentary estate.
"It is absolutely crucial that the government changes course and gives its support to a return to hybrid working."
On Wednesday, Commons Speaker Sir Lindsay Hoyle said it would be up to the government to introduce a motion if they wanted a return to the hybrid Parliament, and for MPs to vote on it.
Shadow leader of the House, Ms Vaz, thanked him for clarifying the process and appealed to the government to take action.
"We are entering a really difficult phase," she said. "As we speak, people are isolating and honourable members are doing the right thing by staying in their constituencies.
"[Meanwhile], the leader has actually scheduled two debates on Covid-19."
She added: "Can I ask [Mr Rees-Mogg] again if we could return to hybrid voting and a remote Parliament? This is a fast moving situation and we have to, and people actually have to, be very careful."
But the Leader of the House said MPs "have a duty to be here doing our business".
Mr Rees-Mogg added: "It is unquestionably the case that democratic scrutiny is essential even during a pandemic.
"We have to be here holding the government to account... so our constituents are represented thoroughly."
He said members had to "ensure we act in a responsible way", pointing to measures taken in the Commons - from social distancing to masks.
But he added: "We have a personal responsibility, we have a duty to legislate, we have a duty to be here.
"We have to show the way and to suggest that democratic accountability is not an essential service seems to me an offence to democracy."
Parliament went on recess a week early after the national lockdown was introduced in March.
While MPs were away from Westminster, the Commons authorities worked on a plan to allow Parliament to function through the pandemic.
In April, MPs signed off on the new "hybrid proceedings", limiting the number of people physically in the chamber, while allowing MPs to participate by video link and vote remotely.
In June, the government wanted to get members back to Westminster, in central London, with Mr Rees-Mogg saying the hybrid Parliament did not allow MPs to properly hold the government to account.
But the government later allowed MPs to self-certify as unable to attend Westminster for medical or public health reasons, and to use the proxy system - where another MP votes on their behalf.
Mr Rees-Mogg's position was criticised by his Tory colleague Dame Cheryl Gillan, who said a "vital part of democracy was being denied to elected MPs by not returning to the initial hybrid system".
She added: "In the face of the rising tide of Covid infections, can we have a debate on the death of democracy in the mother of Parliaments?
"Or will the leader of the House now get off his high horse and remove this restriction that has resulted in the discriminatory silencing of the voices of so many of his colleagues leaving them unable to perform their scrutiny function?"
But fellow Tory, Virginia Crosbie, called on the government to ensure the Commons "remains open in order that each one of us on each side of the House can represent their constituencies".
She added: "We sat during the war. We must sit now at this the most important time for this country."