Keir Starmer says Labour will boycott the commission set up to overhaul the parliamentary sleaze system, after MPs voted to protect the Conservative MP Owen Paterson from suspension over lobbying claims, calling it a “complete and utter sham”.

Boris Johnson threw his weight behind an amendment tabled by former leader of the house Andrea Leadsom, which halts Paterson’s punishment until a new cross-party committee, chaired by John Whittingdale, examines the standards system.

But the Labour leader has said his party will not participate in the committee, throwing into doubt whether the plan can proceed.

In an article for the Guardian, he accused the Conservatives of “wallowing in sleaze”, comparing the situation to an employee of a company found to have misused their position, where “instead of slinging them out, their mates at the company decided to exonerate them and shut down the HR department instead.

“Instead of trying to sort things out, we have a government that wants to stitch things up,” Starmer said. “Their plan is to permanently weaken the structures that hold MPs to high standards. They’ve appointed their own man to oversee the process – a Tory MP who not long ago was the prime minister’s wife’s boss – and gifted themselves a majority on a committee to set the new rules.

“It would be laughable if it wasn’t so serious. The Labour party won’t have anything to do with this complete and utter sham process.”

Despite a significant Tory rebellion in the face of a three-line whip, the amendment passed by 250 votes to 232, after a highly charged debate in the House of Commons.

Johnson earlier insisted he was against paid lobbying, saying he was supporting the Leadsom amendment on the grounds of “natural justice”.

“The issue in this case which involves a serious family tragedy is whether a member of this house had a fair opportunity to make representations in this case, and whether as a matter of natural justice our procedures in this house allow for proper appeal”

The 30-day suspension was recommended after Paterson was found to have repeatedly breached rules on paid lobbying, approaching the government on behalf of two firms that were paying him GBP100,000.

Chris Bryant, the chair of the Commons standards committee, warned Paterson that if Leadsom’s amendment passed, his name would become “a byword for bad behaviour”, and made clear that he had had the opportunity to make his case repeatedly.