Lord Peter Mandelson, the New Labour architect who was sacked as the United Kingdom's ambassador to the United States after messages to Jeffrey Epstein emerged, could be back at work in the House of Lords by Christmas if he chooses to resume his duties, MPs and peers warned on Friday.

Downing Street said the peer's supportive emails to Epstein "materially" changed the calculation over whether he could continue in the post and led to his removal, announced on Thursday. Mandelson has said he regrets continuing a friendship with Epstein after the American financier was convicted in the US.

Mandelson has been on a leave of absence from the House of Lords since the end of January; that leave was granted for the remainder of the current parliamentary session. Under Lords rules, a member on leave who wishes to return must give the Clerk of the Parliaments at least three months' written notice, after which the leave is terminated three months from the date of that notice unless the House directs otherwise. That procedural requirement means he could technically resume attendance and business in the chamber before the year ends.

The possibility of a swift return sparked calls across party lines for formal action. Labour MP Kim Johnson told Sky News that Mandelson should be "removed from the Lords," saying he still "holds the whip" and that losing the whip was the minimum consequence. A senior Labour source in the Lords, however, said Mandelson does not currently hold the party whip because he is on leave of absence.

SNP MP Brendan O'Hara tabled an early-day motion in the Commons accusing Mandelson of bringing Parliament "into disrepute" and called on the government to take legislative steps to remove his peerage. The prime minister's spokesman said whether Mandelson retains his peerage — awarded in 2008 — was "not a matter for me."

Conservative frontbencher Neil O'Brien said Mandelson "cannot return to being a lawmaker and a serving member of the House of Lords" after what he described as attempts to secure an early release for a convicted paedophile, comments that echoed wider demands for a sanctions mechanism. Parliamentary officials said there is not thought to be an existing mechanism to prevent a member on leave from returning without a separate standards process being opened.

Mandelson, a long-standing figure in Labour politics and an architect of the party's modernisation in the 1990s, has been a peer since 2008 and previously served in senior government roles. His removal from the ambassadorial post marks a high-profile exit at a time of renewed scrutiny of public figures' links to Epstein, who was convicted in the United States.

It remained unclear on Friday whether Mandelson would seek to end his leave and return to the Lords, or whether party officials or the House would take steps to prevent a quick resumption of parliamentary duties. Any return would begin with the three-month notice to the Clerk of the Parliaments and could be subject to further action if a standards investigation or other parliamentary measures were launched.