Low-level letterboxes should be banned to prevent postal workers straining their backs or being bitten by dogs, a Conservative MP has said.
Proposing new legislation, just before MPs began debating a no-confidence motion in the government over Brexit, Vicky Ford said it was a "key issue".
She called for all new letterboxes to be installed between 70cm (2ft 3.5ins) and 170cm (5ft 7ins).
The Communication Workers Union is campaigning for new buildings to meet EU letter box height standards.
The CWU, which represents postmen and women, said it did not expect private households or businesses to change their doors immediately, but for the measurements to become a new building regulation in the UK, and to cover replacement doors as well.
The union first started its campaign to raise the level of letterboxes in 1958 and, while it was agreed by the British Standards Agency, it was never enshrined into building standards law.
A similar campaign by its sister union in Ireland saw low-level letterboxes banned in 2001 and the CWU believes "the time has come" to replicate this in the UK.
Moving the bill in the House of Commons, Ms Ford revealed there were 16,800 back-related spells of absence in the Royal Mail last year.
"There are over 95,000 postmen and women working for Royal Mail," she said. "They deliver to 30 million address, they serve each of our communities six days a week, every week of the year, and when I asked postal workers what I could do for them, they asked me to look at low-level letterboxes.
"This bill simply wants to stop developers from building swathes of homes each with a letterbox placed near to the ground and I hope that this will be a moment of unity in British politics."
The bill will come back to the House of Commons for a second reading in March, although it has little chance of becoming law.